obd2 diagnostics

Some scan tools call it theglobal OBD II mode, whileothers describe it as theOBD II generic mode. TheOBD II generic mode allowsa technician to attach his

scan tool to an OBD II-compliant vehi-cle and begin collecting data withoutentering any VIN information into thescan tool. You may need to specificallyselect “OBD II Generic” from the scantool menu. Some scan tools may need asoftware module or personality key be-fore they’ll work in generic OBD II testmode.

The original list of generic data pa-rameters mandated by OBD II and de-scribed in SAE J1979 was short and de-signed to provide critical system dataonly. The useful types of data we can re-trieve from OBD II generic include

short-term and long-term fuel trim val-ues, oxygen sensor voltages, engine andintake air temperatures, MAF or MAPvalues, rpm, calculated load, spark tim-ing and diagnostic trouble code (DTC)count. Freeze frame data and readinessstatus also are available in OBD IIgeneric mode. A generic scan tool alsoshould be able to erase trouble codesand freeze frame data when command-ed to do so.

Data coming to the scan tool throughthe mandated OBD II generic interfacemay not arrive as fast as data sent overone of the dedicated data link connec-tor (DLC) terminals. The vehicle man-ufacturer has the option of using afaster data transfer speed on other DLCpins. Data on the generic interface alsomay not be as complete as the informa-tion you’ll get on many manufacturer-

52 September 2007

OBDII GENERIC PIDDIAGNOSIS

BY KARL SEYFERT

A wealth of diagnostic information is

available on late-model OBD II-compliant

vehicles, even when ‘enhanced’ or

‘manufacturer-specific’ PIDs are not

accessible. It doesn’t take much to use

this information to its best advantage.

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53September 2007

specific or enhanced interfaces. For ex-ample, you may see an engine coolanttemperature (ECT) value in degrees onthe OBD II generic parameter identi-fication (PID) list. A manufacturer-specific data list may display ECT statusin Fahrenheit or Celsius and add a sep-arate PID for the ECT signal voltage.In spite of these and other limitations,OBD II generic mode still containsmany of the trouble codes, freeze framedata and basic datastream informationneeded to solve many emissions-relatedissues.

There are nine modes of operationdescribed in the original J1979 OBD IIstandard. They are:

Mode 1: Show current dataMode 2: Show freeze frame dataMode 3: Show stored trouble codesMode 4: Clear trouble codes and storedvaluesMode 5: Test results, oxygen sensorsMode 6: Test results, noncontinuouslymonitoredMode 7: Show pending trouble codesMode 8: Special control modeMode 9: Request vehicle information

Modes 1 and 2 are basically identical.Mode 1 provides current information,Mode 2 a snapshot of the same datataken at the point when the last diag-nostic trouble code was set. The excep-tions are PID 01, which is available only

in Mode 1, and PID 02, available onlyin Mode 2. If Mode 2 PID 02 returnszero, then there’s no snapshot and allother Mode 2 data is meaningless. Vehi-cle manufacturers are not required tosupport all modes. Each manufacturermay define additional modes aboveMode 9 for other information.

Most vehicles from the J1979 era sup-ported 13 to 20 parameters. The recentphase-in of new parameters will makeOBD II generic data even more valu-able. The California Air ResourcesBoard (CARB) revisions to OBD IICAN-equipped vehicles have increasedthe number of potential generic param-eters to more than a hundred. Not allvehicles will support all PIDs, and thereare many manufacturer-defined PIDsthat are not included in the OBD IIstandard. Even so, the quality andquantity of data have increased signifi-cantly. For more information on the newPIDs that were added to 2004 and laterCAN-equipped vehicles, refer to BobPattengale’s article “Interpreting Gener-ic Scan Data” in the March 2005 issue ofMOTOR. A PDF copy of the article canbe downloaded at www.motor.com.

Establish a BaselineIf you’re repairing a vehicle that hasstored one or more DTCs, make sureyou collect the freeze frame data beforeerasing the stored codes. This data can

54 September 2007

OBD II GENERIC PID DIAGNOSIS

Here’s a basic scanner display showing OBD II genericPIDs. Slow-changing PIDs like IAT and ECT can be fol-lowed fairly easily in this format, but it’s difficult to spotglitches in faster moving PIDs like Spark Advance.

This scan tool also allows the user to graph some PIDs,while continuing to display the others in conventionalnumeric format. Due to OBD II’s refresh capabilities onsome vehicles, it’s best to limit your PID choices tothose directly related to your diagnostic approach.

This photo illustrates how far PID data collection and display have come. Severalhundred thousand techs are still using the original Snap-on “brick” (on the left),which displays a limited amount of PID data on its screen. Scrolling up or downrevealed more PIDs. The color version on the right brought graphing capability tothe brick, and extended the product’s life span by several years.

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be used for comparison afteryour repairs. The “before”freeze frame shot and its PIDdata establish the baseline.

As you begin your diagno-sis, correct basic problemsfirst—loose belts, weak bat-teries, corroded cables, lowcoolant levels and the like.The battery and charging sys-tem are especially important,due to their effect on vehicleelectronics. A good battery, aproperly functioning alterna-tor and good connections atpower and ground circuitsare essential. You can’t as-sume that OBD II will detecta voltage supply problem thatcan affect the entire system.If you have an intermittentproblem that comes andgoes, or random problemsthat don’t follow a logical pattern, checkthe grounds for the PCM and any othercontroller in the vehicle.

If the basics check out, focus your di-agnosis on critical engine parametersand sensors first. Write down what youfind; there’s too much information tokeep it all in your head. Add any infor-mation collected from the vehicle own-er regarding vehicle performance. Jot

down the battery voltage and the resultsof any simple tests, such as fuel pressureor engine vacuum. Look at the Readi-ness Status display to see if there areany monitors that aren’t running tocompletion.

Datastream AnalysisTake your time when you begin lookingat the live OBD II datastream. If you se-lect too many items at one time, the scantool update will slow. The more PIDsyou select, the slower the update ratewill be. Look carefully at the PIDs andtheir values. Is there one line of data thatseems wrong? Compare data items toone another.

Do MAP and BARO agree key on,engine off (KOEO)? Are IAT and ECTthe same when the engine is coldKOEO? The ECT and IAT should bewithin 5°F of each other. ECT shouldreach operating temperature, preferably190°F or higher. If the ECT is too low,the PCM may richen the fuel mixture tocompensate for a (perceived) cold-engine condition. IAT should read ambi-ent temperature or close to underhoodtemperature, depending on the locationof the sensor.

Is the battery voltage good KOEO?Is the charging voltage adequate whenthe engine starts? Do the MAP andBARO readings seem logical? Do the

IAC counts look too high ortoo low? Compare data itemsto known-good values you’dexpect to see for similar op-erating conditions on similarvehicles.

Check short-term fueltrim (STFT) and long-termfuel trim (LTFT). Fuel trimis a key diagnostic parameterand tells you what the com-puter is doing to control fueldelivery and how the adap-tive strategy is operating.STFT and LTFT are ex-pressed as a percentage, withthe ideal range being within±5%. Positive fuel trim per-centages indicate that thepowertrain control module(PCM) is attempting to en-richen the fuel mixture tocompensate for a perceived

lean condition. Negative fuel trim per-centages indicate that the PCM is at-tempting to enlean the fuel mixture tocompensate for a perceived rich condi-tion. STFT will normally sweep rapidlybetween enrichment and enleanment,while LTFT will remain more stable. Ifeither STFT or LTFT exceeds ±10%,this should alert you to a potentialproblem.

56 September 2007


The Snap-on MODIS is a combination scanner, lab/igni-tion scope, DVOM and Troubleshooter. In scanner mode,MODIS can graph several parameters simultaneously,as seen in this screen capture. Remember, althoughthese may look like scope patterns, the reporting ratefor PID data on a scanner isn’t nearly as fast.

When scan tool screen real estate islimited, porting the scan tool into alaptop or desktop PC allows you tograph more PIDs simultaneously.The PC’s much larger memory ca-pacity also makes it possible to col-lect PID data in movie format forlater playback and analysis.

An on-screen description of the PIDdisplayed below the graphing datamay help you to understand whatyou’re looking at, and avoid misunder-standings with measurement units.

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Determine if the condition exists inmore than one operating range. Checkfuel trim at idle, at 1500 rpm and at2500 rpm. If LTFT B1 is 20% at idle butcorrects to 5% at both 1500 and 2500rpm, focus your diagnosis on factors thatcan cause a lean condition at idle, suchas a vacuum leak. If the condition existsin all rpm ranges, the cause is more like-ly to be fuel-related, such as a bad fuelpump, restricted injectors, etc.

Fuel trim can also be used to identifywhich bank of cylinders is causing aproblem on bank-to-bank fuel controlengines. For example, if LTFT B1 is�25% and LTFT B2 is 5%, the sourceof the problem is associated with B1cylinders only, and your diagnosisshould focus on factors related to B1cylinders only.

The following parameters could af-fect fuel trim or provide additional diag-nostic information. Also, even if fueltrim is not a concern, you might find anindication of another problem when re-viewing these parameters:

Fuel System 1 Status and Fuel Sys-tem 2 Status should be in closed-loop(CL). If the PCM is not able toachieve CL, the fuel trim data may notbe accurate.

If the system includes one, the massairflow (MAF) sensor measures theamount of air flowing into the engine.

The PCM uses this information to cal-culate the amount of fuel that should bedelivered to achieve the desired air/fuelmixture. Check the MAF sensor for ac-curacy in various rpm ranges, includingwide-open throttle (WOT), and com-pare it with the manufacturer’s recom-mendations.

When checking MAF sensor read-

ings, be sure to identify the unit of mea-surement. The scan tool may report theinformation in grams per second (gm/S)or pounds per minute (lb/min). Sometechnicians replace the sensor, only torealize later that the scan tool was notset correctly. Some scan tools let youchange the units of measurement fordifferent PIDs so the scan tool matchesthe specification in your reference man-ual. Most scan tools let you switch easilybetween Fahrenheit and Celsius tem-perature scales, for example. But MAFspecs can be confusing when the scantool shows lb/min and we have a specfor gm/S. Here are a few common con-version formulas, in case your scan tooldoesn’t support all of these units ofmeasurement:

Degrees Fahrenheit �� 32 �� 5/9 �� Degrees Celsius Degrees Celsius �� 9/5 + 32 �� Degrees Fahrenheit lb/min �� 7.5 �� gm/Sgm/S �� 1.32 �� lb/min

The Manifold Absolute Pressure(MAP) Sensor PID, if available, indi-cates manifold pressure, which is usedby the PCM to calculate engine load.The reading is normally displayed ininches of mercury (in./Hg). Don’t con-fuse the MAP sensor parameter with in-take manifold vacuum; they’re not thesame. Use this formula: barometric

58 September 2007


Graphs aren’t the only way to display PID data. Oncetransferred to the PC with its greater screen real es-tate, PID data can be converted to formats that relateto the data. A red thermometer scale is much easier tofollow than changing numbers on a scan tool.

PC-based scan tools excel at capturing and displayinglarge amounts of PID data for later analysis. Graphingthe data, then analyzing it on-screen, may allow you tospot inconsistencies and provides an easy method foroverlaying similar or related PID data.

Here’s a peek at some of the addition-al PID data that’s available on late-model vehicles. This screen capturewas taken from a CAN-enabled 2005vehicle, and includes PIDs for EVAPPURGE, FUEL LEVEL and WARM-UPS, aswell as familiar PIDs like BARO. Thismuch PID data in generic mode shouldaid in diagnosis when manufacturer-specific PID data is not available.

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pressure (BARO) � MAP � intakemanifold vacuum. For example, BARO(27.5 in./Hg) � MAP (10.5) � intakemanifold vacuum (17.0 in./Hg). Somevehicles are equipped with only a MAFsensor, some have only a MAP sensorand some are equipped with both.

The PIDs for Oxygen Sensor OutputVoltage B1S1, B2S1, B1S2, etc., areused by the PCM to control fuel mix-ture and to detect catalytic converterdegradation. The scan tool can be usedto check basic sensor operation. Thesensor must exceed .8 volt and drop be-low .2 volt, and the transition from lowto high and high to low should be quick.A good snap throttle test will verify thesensor’s ability to achieve the .8 and .2voltage limits. If this method doesn’twork, use a bottle of propane to manu-ally richen the fuel mixture to check theoxygen sensor’s maximum voltage out-put. To check the sensor’s low voltagerange, simply create a lean conditionand check the voltage.

Remember, your scan tool is not a labscope. You’re not measuring the sensorin real time. The PCM receives the datafrom the oxygen sensor, processes it,then reports it to the scan tool. Also, afundamental OBD II generic limitationis the speed at which that data is deliv-ered to the scan tool. In most cases, thefastest possible data rate is approximate-ly 10 times a second, with only one pa-rameter selected. If you’re requestingand/or displaying 10 parameters, thisslows the data sample rate, and each pa-rameter is reported to the scan tool justonce per second. You can achieve thebest results by graphing or displayingdata from each oxygen sensor separately.If the transition seems slow, the sensorshould be tested with a lab scope to veri-fy the diagnosis before you replace it.

The Engine Speed (RPM) and Igni-tion Timing Advance PIDs can be usedto verify good idle control strategy.Again, these are best checked using agraphing scan tool. Check the RPM,

Vehicle Speed Sensor (VSS) and Throt-tle Position Sensor (TPS) PIDs for ac-curacy. These parameters can also beused as reference points to duplicatesymptoms and locate problems inrecordings.

Most PID values can be verified bya voltage, frequency, temperature,vacuum or pressure test. Enginecoolant temperature, for example, canbe verified with a noncontact temper-ature tester, while intake manifold vac-uum can be verified with an accuratevacuum gauge. Electrical values alsoshould be tested with a DVOM. If theelectrical value exists at the sensor butnot at the appropriate PCM terminal,then the component might be experi-encing a circuit fault.

Calculated ValuesCalculated scan tool values can cause alot of confusion. The PCM may detect afailed ECT sensor or circuit and store aDTC. Without the ECT sensor input,

59September 2007

Circle #31

the PCM has no idea what the coolanttemperature really is, so it may “plug in”a temperature it thinks will work tokeep the engine running long enough toget it to a repair shop. When it doesthis, your scanner will display the fail-safe value. You might think it’s a live val-ue from a working sensor, when it isn’t.

Also be aware that when a compo-nent such as an oxygen sensor is discon-nected, the PCM may substitute a de-fault value into the datastream displayedon the scan tool. If a PID is static anddoesn’t track with engine operating con-ditions, it may be a default value thatmerits further investigation.

Graphing DataIf you’ve ever found it difficult to com-pare several parameters at once on asmall scan tool screen, graphing PIDs isan appealing proposition. Graphingmultiple parameters at the same timecan help you compare data and look forindividual signals that don’t match up toactual operating conditions.

Although scan tool graphing isn’tequivalent in quality and accuracy to alab scope reading, it can provide a com-parative analysis of the activity in thetwo, three, four or six oxygen sensorsfound in most OBD II systems.

Many scan tools are capable of stor-ing a multiple-frame movie of selectedPIDs. The scan tool can be programmedto record a movie after a specific DTCis stored in the PCM. Alternatively, thescan tool movie might be triggeredmanually when a driveability symptomoccurs. In either case, you can observethe data or download it and print it lat-er. Several software programs let youdownload a movie, then plot the valuesin a graphical display on your computermonitor.

Make the Most ofWhat You’ve GotTake the time to learn what your scantool will do when connected to a spe-cific make or model. Do your best togather all relevant information aboutthe vehicle system being tested. Thatway you can get the most out of whatthe scan tool and PCM have to offer.The OBD II system won’t store a DTCunless it sees (or thinks it sees) a prob-lem that can result in increased emis-sions. The only way to know what thePCM sees (or thinks it sees) is to lookthrough the window provided by thescan tool interface.

You have a DTC and its definition.You have freeze frame data that mayhelp you zero in on the affected compo-nent or subsystem. PIDs have alreadyprovided you with additional cluesabout the operation of critical sensors.Keep your diagnosis simple as long asyou can. Now fix the car.

60 September 2007


Visit www.motor.com to downloada free copy of this article.

Circle #32

Circle #33

Circle #34Circle #35

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52 March 2005

If you don’t have a good starting point, driveability diagnosticscan be a frustrating experience. One of the best places to startis with a scan tool. The question asked by many is, “Which

scan tool should I use?” In a perfect world with unlimited re-sources, the first choice would probably be the factory scan tool.

Unfortunately, most techniciansdon’t have extra-deep pockets. That’swhy my first choice is an OBD IIgeneric scan tool. I’ve found that ap-proximately 80% of the driveabilityproblems I diagnose can be narroweddown or solved using nothing morethan OBD II generic parameters. Andall of that information is available onan OBD II generic scan tool that canbe purchased for under $300.

The good news is the recent phase-in

of new parameters will make OBD IIgeneric data even more valuable. Fig. 1on page 54 was taken from a 2002 Nis-san Maxima and shows the typical para-meters available on most OBD II-equipped vehicles. As many as 36 para-meters were available under the originalOBD II specification. Most vehiclesfrom that era will support 13 to 20 para-meters. The California Air ResourcesBoard (CARB) revisions to OBD IICAN-equipped vehicles will increase

the number of potential generic para-meters to more than 100. Fig. 2 on page56 shows data from a CAN-equipped2005 Dodge Durango. As you can see,the quality and quantity of data has in-creased significantly. This article willidentify the parameters that provide thegreatest amount of useful informationand take a look at the new parametersthat are being phased in.

No matter what the driveability is-sue happens to be, the first parame-

53March 2005

INTERPRETING G E N E R I CSCAN DATA

BY BOB PATTENGALE

Readily available ‘generic’ scan data provides anexcellent foundation for OBD II diagnostics.

Recent enhancements have increased the value ofthis information when servicing newer vehicles.

ters to check are short-term fuel trim(STFT) and long-term fuel trim(LTFT). Fuel trim is a key diagnosticparameter and your window into whatthe computer is doing to control fueldelivery and how the adaptive strategyis operating. STFT and LTFT are ex-pressed as a percentage, with the idealrange being within �5%. Positive fueltrim percentages indicate that thepowertrain control module (PCM) isattempting to enrichen the fuel mix-ture to compensate for a perceivedlean condition. Negative fuel trimpercentages indicate that the PCM isattempting to enlean the fuel mixtureto compensate for a perceived richcondition. STFT will normally sweeprapidly between enrichment and en-leanment, while LTFT will remainmore stable. If STFT or LTFT ex-ceeds �10%, this should alert you toa potential problem.

The next step is to determine if thecondition exists in more than one op-

erating range. Fuel trim should bechecked at idle, at 1500 rpm and at2500 rpm. For example, if LTFT B1 is25% at idle but corrects to 4% at both1500 and 2500 rpm, your diagnosisshould focus on factors that can causea lean condition at idle, such as a vacu-um leak. If the condition exists in allrpm ranges, the cause is more likely tobe fuel supply-related, such as a badfuel pump, restricted injectors, etc.

Fuel trim can also be used to identi-fy which bank of cylinders is causing aproblem. This will work only on bank-to-bank fuel control engines. For ex-ample, if LTFT B1 is �20% and LTFTB2 is 3%, the source of the problem isassociated with B1 cylinders only, andyour diagnosis should focus on factorsrelated to B1 cylinders only.

The following parameters could af-fect fuel trim or provide additionaldiagnostic information. Also, even iffuel trim is not a concern, you mightfind an indication of another problem

when reviewing these parameters:Fuel System 1 Status and Fuel

System 2 Status should be in closed-loop (CL). If the PCM is not able toachieve CL, the fuel trim data may notbe accurate.

Engine Coolant Temperature(ECT) should reach operating temper-ature, preferably 190°F or higher. Ifthe ECT is too low, the PCM mayrichen the fuel mixture to compensatefor a (perceived) cold engine condition.

Intake Air Temperature (IAT)should read ambient temperature orclose to underhood temperature, de-pending on the location of the sensor.In the case of a cold engine check—Key On Engine Off (KOEO)—theECT and IAT should be within 5°F ofeach other.

The Mass Airflow (MAF) Sensor,if the system includes one, measuresthe amount of air flowing into the en-gine. The PCM uses this informationto calculate the amount of fuel that

54 March 2005

INTERPRETING GENERIC SCAN DATA

Fig. 1

should be delivered, to achieve thedesired air/fuel mixture. The MAFsensor should be checked for accura-cy in various rpm ranges, includingwide-open throttle (WOT), and com-pared with the manufacturer’s recom-mendations. Mark Warren’s Dec.2003 Driveability Corner column cov-ered volumetric efficiency, whichshould help you with MAF diagnos-tics. A copy of that article is availableat www.motor.com, and an updatedvolumetric efficiency chart is availableat www.pwrtraining.com.

When checking MAF sensor read-ings, be sure to identify the unit ofmeasurement. The scan tool may re-port the information in grams per sec-ond (gm/S) or pounds per minute(lb/min). For example, if the MAFsensor specification is 4 to 6 gm/S andyour scan tool is reporting .6 lb/min,change from English units to metricunits to obtain accurate readings.Some technicians replace the sensor,only to realize later that the scan toolwas not set correctly. The scan toolmanufacturer might display the para-

meter in both gm/S and lb/min to helpavoid this confusion.

The Manifold Absolute Pressure(MAP) Sensor, if available, measuresmanifold pressure, which is used bythe PCM to calculate engine load. Thereading in English units is normallydisplayed in inches of mercury(in./Hg). Don’t confuse the MAP sen-sor parameter with intake manifoldvacuum; they’re not the same. A sim-ple formula to use is: barometric pres-sure (BARO) � MAP � intake mani-fold vacuum. For example, BARO27.5 in./Hg � MAP 10.5 � intakemanifold vacuum of 17.0 in./Hg. Somevehicles are equipped with only aMAF sensor, some have only a MAPsensor and some are equipped withboth sensors.

Oxygen Sensor Output VoltageB1S1, B2S1, B1S2, etc., are used bythe PCM to control fuel mixture. An-other use for the oxygen sensors is todetect catalytic converter degradation.The scan tool can be used to check ba-sic sensor operation. Another way totest oxygen sensors is with a graphing

scan tool, but you can still use the datagrid if graphing is not available onyour scanner. Most scan tools on themarket now have some form of graph-ing capability.

The process for testing the sensorsis simple: The sensor needs to exceed.8 volt and drop below .2 volt, and thetransition from low to high and highto low should be quick. In most cases,a good snap throttle test will verifythe sensor’s ability to achieve the .8and .2 voltage limits. If this methoddoes not work, use a bottle ofpropane to manually richen the fuelmixture to check the oxygen sensor’smaximum output. To check the lowoxygen sensor range, simply create alean condition and check the voltage.Checking oxygen sensor speed iswhere a graphing scan tool helps. Fig.3 on page 57 and Fig. 4 on page 58show examples of oxygen sensor datagraphed, along with STFT, LTFT andrpm, taken from two different graph-ing scan tools.

Remember, your scan tool is not alab scope. You’re not measuring the

56 March 2005


Fig. 2

sensor in real time. The PCM re-ceives the data from the oxygen sen-sor, processes it, then reports it to thescan tool. Also, a fundamental OBDII generic limitation is the speed atwhich that data is delivered to thescan tool. In most cases, the fastestpossible data rate is approximately 10times a second with only one parame-ter selected. If you’re requestingand/or displaying 10 parameters, thisslows the data sample rate, and eachparameter is reported to the scan tooljust once per second. You can achievethe best results by graphing or dis-playing data from each oxygen sensorseparately. If the transition seemsslow, the sensor should be tested witha lab scope to verify the diagnosis be-fore you replace it.

Engine Speed (RPM) and Igni-tion Timing Advance can be usedto verify good idle control strategy.Again, these are best checked using agraphing scan tool.

The RPM, Vehicle Speed Sensor(VSS) and Throttle Position Sensor(TPS) should be checked for accuracy.

These parameters can also be used asreference points to duplicate symptomsand locate problems in recordings.

Calculated Load, MIL Status,Fuel Pressure and Auxiliary InputStatus (PTO) should also be consid-ered, if they are reported.

Additional OBD IIParametersNow, let’s take a look at the more re-cently introduced OBD II parameters.These parameters were added on 2004CAN-equipped vehicles, but may alsobe found on earlier models or non-CAN-equipped vehicles. For example,the air/fuel sensor parameters wereavailable on earlier Toyota OBD II ve-hicles. Fig. 2 was taken from a 2005Dodge Durango and shows many ofthe new parameters. Parameter de-scriptions from Fig. 2 are followed bythe general OBD II description:

FUEL STAT 1 � Fuel System 1Status: Fuel system status will displaymore than just Closed Loop (CL) orOpen Loop (OL). You might find one

of the following messages: OL-Drive,indicating an open-loop conditionduring power enrichment or decelera-tion enleanment; OL-Fault, indicatingthe PCM is commanding open-loopdue to a system fault; CL-Fault, indi-cating the PCM may be using a differ-ent fuel control strategy due to anoxygen sensor fault.

ENG RUN TIME � Time Since En-gine Start: This parameter may beuseful in determining when a particu-lar problem occurs during an enginerun cycle.

DIST MIL ON � Distance TraveledWhile MIL Is Activated: This para-meter can be very useful in determin-ing how long the customer has al-lowed a problem to exist.

COMMAND EGR � EGR_PCT:Commanded EGR is displayed as apercentage and is normalized for allEGR systems. EGR commandedOFF or Closed will display 0%, andEGR commanded to the fully open

57March 2005

Fig. 3

position will display 100%. Keep inmind this parameter does not reflectthe quantity of EGR flow—only whatthe PCM is commanding.

EGR ERROR � EGR_ERR: Thisparameter is displayed in percentageand represents EGR position errors.The EGR Error is also normalized forall types of EGR systems. The readingis based on a simple formula: (ActualEGR Position � Commanded EGR) �Commanded EGR � EGR Error. Forexample, if the EGR valve is command-ed open 10% and the EGR valve movesonly 5% (5% � 10%) � 10% � �50%error. If the scan tool displays EGR Er-ror at 99.2% and the EGR is command-ed OFF, this indicates that the PCM isreceiving information that the EGRvalve position is greater than 0%. Thismay be due to an EGR valve that isstuck partially open or a malfunctioningEGR position sensor.

EVAP PURGE � EVAP_PCT: Thisparameter is displayed as a percent-age and is normalized for all types ofpurge systems. EVAP Purge Control

commanded OFF will display 0% andEVAP Purge Control commandedfully open will display 100%. This isan important parameter to check ifthe vehicle is having fuel trim prob-lems. Fuel trim readings may be ab-normal, due to normal purge opera-tion. To eliminate EVAP Purge as apotential contributor to a fuel trimproblem, block the purge valve inletto the intake manifold, then recheckfuel trim.

FUEL LEVEL � FUEL_PCT: Fuellevel input is a very useful parameterwhen you’re attempting to completesystem monitors and diagnose specif-ic problems. For example, the misfiremonitor on a 1999 Ford F-150 re-quires the fuel tank level to begreater than 15%. If you’re attempt-ing to duplicate a misfire condition bymonitoring misfire counts and the fuellevel is under 15%, the misfire moni-tor may not run. This is also impor-tant for the evaporative emissionsmonitor, where many manufacturersrequire the fuel level to be above15% and below 85%.

WARM-UPS � WARM_UPS: Thisparameter will count the number ofwarm-ups since the DTCs were cleared.A warm-up is defined as the ECT risingat least 40°F from engine starting tem-perature, then reaching a minimumtemperature of 160°F. This parameterwill be useful in verifying warm-up cy-cles, if you’re attempting to duplicate aspecific code that requires at least twowarm-up cycles for completion.

BARO � BARO: This parameter isuseful for diagnosing issues withMAP and MAF sensors. Check thisparameter KOEO for accuracy relat-ed to your elevation.

C AT TMP B1S1 /B2S1 �CATEMP11, 21, etc.: Catalyst tem-perature displays the substrate temper-ature for a specific catalyst. The tem-perature value may be obtained directlyfrom a sensor or inferred using othersensor inputs. This parameter shouldhave significant value when checkingcatalyst operation or looking at reasonsfor premature catalyst failure, say, dueto overheating.

58 March 2005


Fig. 4

CTRL MOD (V) � VPWR: I wassurprised this parameter was not in-cluded in the original OBD II specifi-cation. Voltage supply to the PCM iscritical and is overlooked by manytechnicians. The voltage displayedshould be close to the voltage presentat the battery. This parameter can beused to look for low voltage supply is-sues. Keep in mind there are othervoltage supplies to the PCM. The igni-tion voltage supply is a common sourceof driveability issues, but can still bechecked only with an enhanced scantool or by direct measurement.

ABSOLUT LOAD � LOAD_ABS:This parameter is the normalized valueof air mass per intake stroke displayedas a percentage. Absolute load valueranges from 0% to approximately 95%for normally aspirated engines and 0%to 400% for boosted engines. The infor-mation is used to schedule spark andEGR rates, and to determine thepumping efficiency of the engine for di-agnostic purposes.

OL EQ RATIO � EQ_RAT: Com-manded equivalence ratio is used to de-termine the commanded air/fuel ratioof the engine. For conventional oxygensensor vehicles, the scan tool should dis-play 1.0 in closed-loop and the PCM-

commanded EQ ratio during open-loop. Wide-range and linear oxygensensors will display the PCM-com-manded EQ ratio in both open-loopand closed-loop. To calculate the actualA/F ratio being commanded, multiplythe stoichiometric A/F ratio by the EQratio. For example, stoichiometric is a14.64:1 ratio for gasoline. If the com-manded EQ ratio is .95, the command-ed A/F is 14.64 � 0.95 � 13.9 A/F.

TP-B ABS, APP-D, APP-E, COM-MAND TAC: These parameters relateto the throttle-by-wire system on the2005 Dodge Durango of Fig. 2 and willbe useful for diagnosing issues with thissystem. There are other throttle-by-wiregeneric parameters available for differ-ent types of systems on other vehicles.

There are other parameters of inter-est, but they’re not displayed or avail-able on this vehicle. Misfire data will beavailable for individual cylinders, similarto the information displayed on a GMenhanced scan tool. Also, if available,wide-range and linear air/fuel sensorsare reported per sensor in voltage ormilliamp (mA) measurements.

Fig. 5 above shows a screen capturefrom the Vetronix MTS 3100 Mas-tertech. The red circle highlights the“greater than” symbol (>), indicatingthat multiple ECU responses differ in

value for this parameter. The blue cir-cle highlights the equal sign (=), indi-cating that more than one ECU sup-ports this parameter and similar valueshave been received for this parameter.Another possible symbol is the excla-mation point (!), indicating that no re-sponses have been received for thisparameter, although it should be sup-ported. This information will be usefulin diagnosing problems with data onthe CAN bus.

As you can see, OBD II generic datahas come a long way, and the data canbe very useful in the diagnostic process.The important thing is to take time tocheck each parameter and determinehow they relate to one another.

If you haven’t already purchased anOBD II generic scan tool, look forone that can graph and record, if pos-sible. The benefits will immediatelypay off. The new parameters will takesome time to sort out, but the diag-nostic value will be significant. Keepin mind that the OBD II genericspecification is not always followed tothe letter, so it’s important to checkthe vehicle service information forvariations and specifications.

60 March 2005


Fig. 5


37August 2008

Last month’s installmenton datastream analysisfocused on the value offreeze frame data, Mode5 and Mode 6 data andKOEO (key on, engine

off) datastream. This month’s discussionpicks up where we left off, with KOER(key on, engine running) analysis. So goahead, start the engine!

I recommend that KOER data col-lection always start in the generic, orglobal OBD II interface. Why? Becausegeneric datastream PID values are nev-er substitutes for actual sensor readings.For example, you can disconnect theMAP sensor connector on a Chrysler

product and drive it around while moni-toring datastream in the enhanced(manufacturer-specific) interface. (Trythis yourself; don’t just take my word forit.) You’ll see the MAP PID changealong with the TPS sensor reading andrpm, showing a range of values that re-flect likely MAP readings for each con-dition, moment by moment. These aresubstituted values. If you looked at theMAP voltage PID, however, it wouldshow an unchanging reference voltage.In the enhanced interface, substitutionscan and do occur. But in the generic in-terface, substituted values are never al-lowed. You would see MAP shown at aconstant pressure equal to something a

DATASTREAM IN-DEPTH ANALYSIS

BY SAM BELL

We began this two-part article with a

discussion of preliminary OBD II datastream

analysis, conducted with the engine off.

We’re going deeper this time, to explain

the value of datastream information

collected with the engine running.

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bit higher than BARO. The generic in-terface allows calculated values, butnever substituted values.

So, what are we looking for, now thatwe’ve finally started the engine? Thespecific answer, of course, will dependlargely on the details of the customercomplaint and/or DTC(s) that arestored. We might, for example, be focus-ing on fuel trim numbers (and trends) ifour code suggests an underlying air/fuelmetering problem. We might be lookingmost closely at engine coolant tempera-ture, and time-until-warm measure-ments when that seems warranted. Per-haps our problem lies in the evap area,or involves EGR flow. But ultimately, itdoesn’t matter what the specific issue is;we’ll have to focus in on the systemic in-teractions that determine the overallcharacteristics of a particular data set.

Here’s a concrete example to illus-trate what I mean. The vehicle in ques-tion is a 1999 Chevy Venture minivanwith the 3.4L V6. There was a DTC

P0171 (Exhaust Too Lean, Bank 1) inmemory with an active MIL. The sumof Short Term and Long Term FuelTrims in freeze frame was in excess of

50%. Fuel pressure and volume hadbeen verified as within specification.

When evaluating a fuel trim troublecode, one of the first steps must alwaysbe to verify that the oxygen sensor (onwhich the DTC is based) is functioningcorrectly. During the test drive, I ob-served the O2 sensor switching rich, butnot as often as would be expected if thevery large fuel trim corrections shownwere actually effective. Indeed, on theface of it, datastream seemed to confirmthe DTC. Longtime readers, however,can probably anticipate what my nexttests were: I checked the actual lambdavalue of the exhaust gases. Then Ilooked for a dynamic response as I artifi-cially enriched the system with a blast ofpropane, then enleaned it by discon-necting a major vacuum hose. (See“What Goes In…Harnessing Lambda asa Diagnostic Tool” in the September2005 issue of MOTOR. Search the indexat www.motormagazine.com for all MO-TOR magazine articles mentioned.) Hav-

38 August 2008


Data collection and analysis might yield somehelpful information, if you can find the wheatwithin the chaff. This is only a small portion of alarger data set with 100 values per PID.

When evaluating afuel trim troublecode, one of thefirst steps must

always be to verifythat the oxygen

sensor (on which theDTC is based) is

functioning correctly.

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39August 2008

ing found the idle lambda at a ridicu-lously low value of .85 (indicating a mix-ture with 15% more fuel than needed), Iwas not surprised to see that the O2 sen-sor didn’t register a rich condition untilthe engine was very nearly flooded withpropane. When I removed the purgehose, engine rpm climbed and the en-gine smoothed out, while lambdamarched toward the stoichiometric idealvalue of 1.00. Once the faulty O2 sensorwas replaced, all aspects of driveabilityimproved, and the minivan returned toits previous fuel consumption levels.

Dynamic tests verify DTC accuracy.In some instances, we may be able toutilize bidirectional controls embeddedwithin our scan tool packages to actuatevarious components. In other cases, wemay need to improvise, using signalsimulators, power probes, jumpers,propane or just good, old-fashioned testdriving as required to initiate changewithin the system we’re working on.(I’m not saying that it will always be as

easy as it was with the Venture. You andI know there will be problems that don’tset DTCs, problems that do set DTCsthat have no apparent connection to the

actual root fault and, of course, prob-lems that set appropriate codes yet arestill really hard to diagnose.)

Floodlights and SpotlightsOne of the most powerful features ofmost scan tools is, as nearly as I cantell, one of the least used. This is theso-called flight recorder, data logger ormovie mode. By whatever name it’sknown, this is an analytical tool of con-siderable value.

Take a look at the portion of savedscan data portrayed in the chart on page38. As you see, any value in that infor-mation is well hidden. This might betermed a “floodlight” view, showing toomany values for too many parameters.But look at the “spotlight” view above,where I’ve selected and graphed a fewof the same PID values. This was a ve-hicle where there was no DTC stored inmemory. By including both upstreamO2 sensors, I have provided myself across-check, as there is less likelihood of

Graphical representations of scan data “movies” can speed analysis. As an added bonus,using your scanner’s flight recorder mode allows you to concentrate on your driving. Thedata set here clearly points to a lack of adequate fuel volume. This graphical representa-tion is derived from the exact same movie capture seen in the chart on the previous page.

One of the mostpowerful features

of most scan tools—the so-called flightrecorder—seems tobe one of the leastused. But it’s ananalytical tool of

considerable value.

both being bad. Similarly, MAF andrpm track nicely with one another, againproviding a good cross-check. The datavalues at the cursor (the vertical line atframe 2) are called out at the left side ofeach PID’s plot. The upstream O2 sen-sors are switching nicely at 2000 rpm (asshown at frame �51), but the graphicinterface reveals an obvious problem at

higher speeds as the O2 sensors flat-linelean. A new fuel pump restored themissing performance.

Slow Motion andHigh SpeedMoviemakers speed up or slow downthe action on the screen by shooting atdifferent numbers of frames per sec-

ond. When film shot at 20 frames persecond is played back at 60 frames persecond, the action seems to be occur-ring at three times the speed. Just as a56k dial-up modem is slower than aDSL Internet connection, scan datatransfer rates also vary according to theinterface used. Generic communica-tion modes often travel at a crawl, es-

40 August 2008


Most MOTOR readers have at least a passing fa-miliarity with the concept of OBD II monitor

completion status. Even so, a brief refresher may bein order. OBD II monitors are simply formalized setsof self-tests all related to a particular system orcomponent.

CCoonnttiinnuuoouuss mmoonniittoorrss.. With a few very rare ex-ceptions (mostly for 1998 and earlier models), theso-called continuous monitors always show up as“complete,” “done” or “ready.” Take this status re-port with a grain of salt. Unplug the IAT sensor, startthe engine and check that the “ComprehensiveComponent Monitor” readiness status shows com-plete. Is the MIL on? Are there any pending codes?How long would you have to let the vehicle idle be-fore it will trip the MIL and show a P0113 (IAT Sen-sor Circuit Voltage High) DTC?

As it turns out, depending on the specific make,model and powertrain package, there are severalspecific criteria that must be met before the codewill set. In one instance, the PCM must detect a VSSsignal of 35 mph or more and an ECT value of 140°For more, the calculated IAT must be less than �38°Fand all of these conditions must be met for at least180 seconds of continuous duration, during whichno other engine DTCs are set—all while MAF is lessthan 12 grams per second. (This particular example,incidentally, is a two-trip code. Some other manu-facturers may make this and other DTCs under thecomponent monitor’s jurisdiction into one- or two-trip codes, sometimes with even more complicatedentry criteria.)

Continuous monitors include the comprehensivecomponent monitor, the fuel monitor and the mis-fire monitor. Each monitor runs continuously whenconditions are appropriate, but not during all actu-al driving. For example, the misfire monitor is oftensuspended during 4WD operation, since feedbackthrough the axles over rough roads might causeuneven disruption of the CKP signals, which could

otherwise be misidentified as misfires. Similarly, ex-tremely low fuel tank levels may suspend both mis-fire and fuel system monitors to avoid setting aDTC for running out of gas.

NNoonnccoonnttiinnuuoouuss mmoonniittoorrss.. As I pointed out lastmonth, it’s important to note the readiness status ofthe other, noncontinuous monitors as well. These arethe monitors whose status will change to “incom-plete,” “not ready” or “not done” when the codesare cleared. If a vehicle arrives at your shop showingone or more incomplete monitors, it’s likely thatsomeone has already cleared the codes before it gotto you. (There are a few vehicles—for example, some1996 Subarus—which may reset monitor status to in-complete at every key-off, or other vehicles whichmay have certain monitors which cannot be made torun to completion in normal driving, such as the evapmonitor on some Toyota Paseos.) If a vehicle showsup with incomplete monitors, however, you shouldcertainly document that fact on your work order andbe sure to advise the customer that there’s a very realpossibility that one or more other codes may recur af-ter the current repair has been completed. For moreon this subject, see my article “How Not to Get MIL-Stoned” in the April 2004 issue of MOTOR.

More importantly, for our present purposes, theexistence of incomplete monitors means that youmay not be getting the whole picture as to whatails the vehicle you’re looking at. Keep an openmind, remembering that there may be other, as yetunknown issues hidden behind that incompletemonitor, and try not to rush your diagnosis. Asmentioned in last month’s installment, there maybe some valuable data accessible via Mode 6 evenif the monitor is not complete, but there is a veryreal possibility that Mode 6 data for any incom-plete monitor may turn out to be unreliable. And,of course, don’t overlook any pending DTCs. Re-member, these do not illuminate the MIL, so youmust seek them out on your own.

Monitors 1.01

42 August 2008


pecially in comparison to CAN speeds.If you’re stuck with a generic interface,you can often accomplish more bylooking at less.

The key here is PID selection.Choose the smallest number of PIDsthat will give you the information you

actually need. Three or four are usu-ally sufficient. This is your version ofthe filmmaker’s high-speed actiontrick, as you get more updates perunit time the fewer PIDs you select.With several hundred possible PIDsfrom which to choose, it’s just too

easy to miss an intermittent dataglitch, or to drown in a sea of toomuch information (see “Live Data vs.‘Live Data’” on page 44).

Most MOTOR readers are familiarwith the ways in which some of the ma-jor OEMs have organized data PIDsfor display in their enhanced scan toolinterfaces. Groupings such as Misfire,Driveability, Emissions, Accessoriesand the like are good examples of thetypes of data sets you may want to con-struct while analyzing different sorts ofproblems. Tracking down a nasty inter-mittent problem? Don’t hesitate topare down the OEM groupings evenfurther to speed data updates.

Code-Setting Criteria andOperating ConditionsIf we’re trying to resolve a MIL-oncomplaint, it’s critical that we first re-view both the exact code-setting crite-ria and the operating conditions as re-vealed in our previously recordedfreeze frame data. We’ll need to drivein such a way as to complete a good“trip” so the affected monitors canrun to completion. (For a more de-tailed discussion of OBD II trips andmonitors, see “Monitors 1.01” onpage 40.) If we fail to meet the condi-tions under which the self-test (moni-tor) will run, we cannot hope to makeprogress. Using the previously record-ed freeze frame parameters gives us agood general idea of the operatingconditions required. Merely duplicat-ing speed, load, temperature and oth-er basic characteristics may not beenough. This is why we need to re-view and understand the details of thecode-setting criteria and the monitor’sself-test strategy. For example, somemonitors cannot run until others havealready reached completion. A typicalexample would be a catalytic convert-er monitor that is suspended until theoxygen sensor monitors have run andpassed.

Some trouble codes, or even pendingcodes, suspend multiple monitors. Oth-er vehicle faults may then go undetect-ed until all monitors can run again. AP0500 (VSS Malfunction) in a Corolla,for example, will effectively suspendeven the misfire monitor.

It seems like a no-brainer: When you’re done with allyour diagnostic tests and you’ve made the necessary

repairs, you should turn off the MIL, right? That’swhat your customer probably expects, and as we allknow, meeting customer expectations is an importantpart of running a successful business.

But there are often times when you should leavethe MIL on. If your area uses an OBD II “plug & play”emissions test, the regulations usually require that nomore than one monitor can be incomplete as of thetime of testing for model year 2001 and newer vehi-cles, with no more than two incomplete monitors for1996 to 2000 models. In some areas, retest eligibilityrequires that the converter monitor must show “com-plete” before a retest is valid.

If an emissions test or retest is looming in your cus-tomer’s future, you and he must work out the prosand cons of clearing the codes and resetting the mon-itors to “incomplete.” If you clear the codes, the mon-itors will reset as well. This will require that someonewill have to drive a sufficient number of monitors tocompletion before a retest will be valid. If localweather conditions, for example, will prevent themonitors from running in a timely way, your customermight be better off if you leave the MIL on. Then yourcustomer would have to drive only those portions ofthe drive trace needed to run the monitor underwhich the current DTC set.

For example, if you’re in the frigid climes of an up-per Midwestern winter and a customer’s vehicle failedan emissions test because of a faulty O2 sensor heater,you’ll both be ahead if you don’t clear the code, lettingit expire naturally as the heater monitor runs success-fully to completion on the next two trips. This willavoid the necessity of rerunning all the rest of themonitors. Of course, if the vehicle failed the evap mon-itor, you’ll be better off clearing the code, because pro-longed subfreezing temperatures may make runningthat particular monitor successfully virtually impossiblefor weeks at a time.

Lights Out?

The net result is that we may have to clear the currentDTCs and extinguish the MIL before our test drive canbear fruit. (But again, please be sure to read and recordall the freeze frame data, the status of all monitors, the listof both current and pending DTCs and any availableMode 6 data before clearing the MIL (see “Lights Out?”on page 42).

We’ll need to drive long enough to let the monitors inquestion reach completion. In some cases, this may requirean extended period of time. Many Ford products, for exam-ple, normally require a minimum of a six-hour cold-soak be-fore the evap monitor can run, although there may be ways toforce this issue in some instances. Many Chrysler oxygen sen-sor monitors run only after engine shut-down (with key off),

so that no amount of driving will ever bring them to comple-tion. Certain monitors, and apparently even certain scan tools,may require a key-off sequence before the monitor status willupdate from incomplete to complete. MOTOR offers an excel-lent resource to help you understand these details—the OBDII Drive Cycle CD Version 7.0, available from your localMOTOR Distributor (1-800-4A-MOTOR).

In some cases, local weather conditions may make monitorcompletion seem impossible until a later date, usually be-cause of ambient temperature requirements, although some-times as a result of road conditions. In most cases, however, itwill still be possible to complete the monitor by running thevehicle on a lift or dynamometer. This option may occasional-ly result in setting, say, an ABS code, but most monitors canbe run to completion swiftly and successfully on a lift. Thisoption may also offer a safer, faster alternative to actual driv-ing, as trees and telephone poles are less likely to jump infront of a vehicle on a stationary lift.

44 August 2008

DATASTREAM ANALYSIS

Circle #22

Intermittent interruptions of sensor datacan cause tricky driveability problems.

Some glitches may set a DTC while othersmay not. While viewing datastream mayreveal an intermittent sensor problem, itshould not be relied upon to do so. The is-sue, once again, is in the data rate. Even amoderately fast interface, say the 41.6kbps (kilobytes per second) J-1850 PWMused on many Ford products, can easilymiss a several-millisecond dropout if it’snot that particular PID’s turn in the data-stream. Where symptoms or DTCs point to-ward an intermittent sensor glitch, you’reprobably better off breaking out yourscope or graphing multimeter.

Live Data vs. ‘Live Data’

ConclusionsProper in-depth datastream analysiscan often light the way toward correctdiagnosis of driveability concerns.Recording all available DTCs, pendingDTCs, freeze frame data and Mode 5and Mode 6 results before clearing anyDTCs is essential. Specific setting cri-teria for each DTC are manufacturer-determined, regardless of whether thecode assigned is generic or manufac-turer-specific. Freeze frame data setscan be used to recreate the operatingconditions under which a previous fail-ure occurred and can help illuminatethe conditions under which certainself-tests are conducted. Mode 5 andMode 6 test results can help in analyz-ing the type and extent of certain fail-ures. KOEO datastream analysis cansometimes reveal sensor faults or ration-ality concerns that might otherwise beoverlooked.

Looking at KOEO and KOER data-stream on a regular basis makes known-good values familiar. Once you know

the correct values, the conditions ac-companying problems identified byfreeze frame are easier to spot. KOERdata can highlight current problems, es-

pecially when used in conjunction withgraphical scanner interfaces. Genericdata PIDs cannot include substitutedvalues, and so may point up faults easilyoverlooked in more enhanced inter-faces. Careful selection of custom-grouped PIDs can provide faster scan-ner update rates.

Pick your tools wisely. To verify hardfaults, monitor datastream as you runactuator tests. Look for any mismatchbetween the command sent to a com-ponent and its actual response. For in-termittent problems, record and graphdata. In tough cases, test circuits withyour scope or meter to verify actualvoltage for comparison to specs.

Used properly, these techniqueswill help you arrive quickly and confi-dently at an accurate diagnosis of theroot cause of most driveability com-plaints.

45August 2008

Circle #23

When trying toresolve a MIL-oncomplaint, it’scritical to first

review the exactcode-setting

criteria and theoperating

conditions asrevealed in the

freeze frame data. This article can be found online at

www.motormagazine.com.

Once in a while we mayencounter a total fail-ure of a MAF sensor,one that is, perhaps,short circuited or inter-nally open. Much more

common, however, are failure modes inwhich the MAF sensor has become un-reliable, underreporting or overreport-ing the true airflow into the engine. In-deed, as we shall see, many MAF sen-sor failures actually result in both un-derreporting and overreporting!

Before we get down to brass tacks, abrief review of the basics of MAF sys-tems is in order. Fuel control systemsfor most modern gasoline engines arecentered either on MAF or MAP (man-ifold absolute pressure). MAF systems,which, as their name suggests, measurethe weight of incoming air and thenmeter the appropriate amount of fuel toensure efficient combustion, are poten-tially more precise, although MAP sys-tems, which calculate fuel requirementsbased on engine load, have historicallydemonstrated greater reliability.

As you already know, combustion ismost efficient when the ratio of air tofuel is approximately 14.7:1 by weight.Mass and weight are essentially synony-mous in the presence of a sufficientlystrong gravitational field such as theEarth’s. Thus, knowing the weight ofthe air entering the engine allows theengine controller to meter the exactamount of fuel required to achieve effi-cient combustion. The controller com-mands the fuel injectors to open for anamount of time calculated to be suffi-cient to allow the correct weight of fuelto enter the engine, providing that thefuel’s pressure is known. Fuel delivery isfine-tuned by applying fuel trim correc-tions derived from the closed-loop feed-back of the oxygen sensor(s).

If the entire system is working as de-signed, fuel trim corrections, expressedas a percentage deviation from the basefuel delivery programming, will be with-in 10% (either positive or negative) ofthe programmed quantity. In the ab-sence of a MAF-specific diagnostic trou-ble code (DTC), what would first leadus to even suspect that a faulty MAFsensor might underlie a particular drive-ability problem?

To function correctly, all of the air

entering an engine’s combustion cham-bers must be “seen” by the MAF sen-sor. This means that any vacuum or airleak downstream of the sensor will re-sult in insufficient fuel metering, caus-ing a lean condition in open-loop opera-tion and higher-than-normal fuel trimvalues in closed-loop. When we en-counter a MAF sensor-equipped vehi-cle exhibiting these symptoms, we need

to check for unmetered airflow first.Remember, too, that unmetered airflowmay not require an external air leak. Anincorrectly applied or faulty PCV valvecan result in incorrect MAF data wherethe PCV intake through the breatherhose is upstream of the MAF.

So, the first two rules of MAF sensordiagnosis are:

1. Find and eliminate all external air

28 July 2006

MAF DIAGNOSIS

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SUCCESSFUL

MAF SENSORDIAGNOSIS

BY SAM BELL

A broad range of seemingly unrelated or

contradictory driveability complaints

may arise from MAF sensor

performance faults. Use this guide to

navigate out of a diagnostic thicket or,

better still, to avoid one entirely.

or vacuum leaks downstream of theMAF sensor. When in doubt, use asmoke machine, or lightly pressurizethe intake manifold and spray with asoap & water solution.

2. Verify that the manufacturer-speci-fied PCV valve is correctly installed andfunctioning as designed. (This is one in-stance where precautionary replace-ment may be cost-justified.)

Only after these two steps have beencompleted can you safely proceed withother diagnostics. The foremost cluethat the fault lies with the MAF sensoritself will be excessive fuel trim correc-tions, usually negative at idle, more orless normal in midrange operation andpositive under high load conditions (see“How Contamination Affects Hot-Wire& Hot-Film MAF Sensors” on page 32).

While there are several distinct MAFsensor technologies ranging from hot-wire or hot-film to Karman vortex andCorialis sensors, and while MAF sensoroutputs may take the form of variablefrequency, variable current or a simpleanalog voltage, the diagnostic principlesremain largely the same.

Let’s start with Ford vehicles, for acouple of reasons. First, they are sowidespread that most of us are familiarwith them. Second, most MAF sensor-equipped Ford products make use of aPID (Parameter IDentification) calledBARO (barometric pressure). Up to2001 models, this was an inferred, orcalculated, value generated by the PCM(powertrain control module) in re-sponse to the maximum MAF flowrates observed on hard wide-openthrottle (WOT) acceleration. Wherethis calculated BARO PID is available,it is of great diagnostic value, since itcan confirm MAF sensor accuracy, ifonly under high flow rate conditions.

To use the BARO PID, you mustfirst know your approximate local baro-metric pressure. You might consult theBARO PID on a known-good MAPsensor-equipped vehicle. Alternatively,your local airport can provide this data.Do not rely on local weather stations,however, since these usually report a“corrected” barometric pressure. Ifweather information is the only avail-able source, a rule of thumb is to sub-tract about 1 in. of mercury (1 in./Hg)for every 1000 ft. of elevation above sealevel. This will yield a rough estimate ofyour actual local barometric pressure.For greater accuracy, you can purchasea functional barometer for somethingless than $40. Compare this data withthe BARO PID. A large discrepancyhere—say, more than 2 in./Hg—shoulddirect your suspicions toward the MAF.

Confirm your hypothesis as follows:First, make sure you have followed thesteps outlined in the two rules above.Next, record all freeze frame data andall DTCs, including pending DTCs. Ifthe OBD monitor readiness status foroxygen sensors shows READY, proceedto the next step. If it doesn’t, refer tothe procedures in the following para-graph now. Next, perform a KAM(Keep Alive Memory) reset and drivethe vehicle. Make sure your test drive

29July 2006

includes at least three sustained WOTaccelerations. (It’s not necessary tospeed to accomplish a sustained WOTacceleration. Rather than a WOT snapfrom idle, an uphill downshift at 20 to30 mph is usually sufficient. The WOTprescription can be met at throttleopenings as low as 50% to 70%.) TheBARO PID should update from its de-fault reading by the end of the thirdWOT acceleration. If it’s now close toyour local barometric pressure, theMAF sensor is not likely to be faulty. IfBARO is not close, try one of the clean-ing techniques explained in the sidebar“Keeping It Clean” on page 34, thenagain reset KAM and take a test drive. Ifthe BARO is still out of range, a replace-ment MAF sensor is in your customer’sfuture. Unfortunately, in many 2002 andlater Fords, the calculated BARO PID issupplanted by a direct BARO reading

taken from a sensor incorporated intothe ESM (EGR System Management)valve, greatly lessening its diagnostic val-ue for our current purposes.

If the oxygen sensor monitor statusshowed INCOMPLETE above, you’llhave to verify O2 sensor accuracy andperformance before performing theKAM reset procedure. Use a 4- or 5-gasanalyzer to determine whether theair/fuel ratio is correct in closed-loopoperation. The notes about lambda (�)below should help.

Outside of the Ford family, MAFsensor diagnosis is more difficult. Largefuel trim corrections—either positive ornegative—are often the only initialpointer to MAF sensor problems.Again, any and all air leaks downstreamof the MAF sensor must be repairedfirst. Since accurate fuel trim correc-tions depend on correct O2 sensor out-

puts, you must verify the functionalityof these sensors first. The easiest andfastest way to do this is by checkinglambda, a type of measure of the air/fuelratio. (For a detailed explanation, seemy article in the September 2005 issueof MOTOR.) If the O2 sensors are func-tioning correctly, lambda at idle shouldbe very nearly equal to 1.00 in closed-loop. You may wish to check this also at1500 to 1800 rpm to verify adequatemixture control off idle. Once lambda isfound to be correct, the O2 sensors areproven good. Then any fuel trim adjust-ments must result from unmetered orincorrectly metered airflow or from in-correct fuel delivery.

Distinguishing between fuel deliveryproblems and MAF sensor problemscan be very frustrating. Start by verify-ing fuel pressure and volume. (Thosewho rely on pressure alone may regret

30 July 2006

SUCCESSFUL MAF SENSOR DIAGNOSIS

Fig. 1 Fig. 2

Fig. 3 Fig. 4

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it.) Use your scan tool to record criticaldata PIDs and graph them for analysis.Here are a couple of examples:

In Fig. 1 on page 30, taken during aperiod of closed-loop operation, short-term fuel trims (blue and green traces)for each bank were above 13% at 1100rpm (red trace), yet dropped sharplynegative at 3600 rpm, proving that inade-quate fuel delivery was not the problem.The values indicated in the legend box-es correspond to the readings obtained

at the indicated cursor position (verticalblack line). The vertical white line indi-cates the trigger point for the recording.Subsequent diagnostics focused on theMAF sensor and the PCV system.

Take a look at the scan data graphshown in Fig. 2. It shows a car whosefaulty fuel pump was unable to deliversufficient fuel under high load condi-tions. Notice the very low O2 sensorreadings (displayed in blue) corre-sponding to the cursor (black vertical

line just to the right of the zero timestamp). Fuel pressure was within specat idle and at about 2000 rpm, but vol-ume was very low. The sudden drop-off in O2 activity in response to hardacceleration is a characteristic ob-served in many instances of MAF sen-sor faults as well.

Ultimately, known-good snapshots,waveforms and other data sets are in-valuable. Take a look at the scan snap-shot in Fig 3. Does it show good fueltrim and appropriate MAF sensorreadings?

Since total fuel trim stays well withinthe 0 ±10% range throughout thetrace, it’s a good bet that the MAF sen-sor is working well, at least under thesampled conditions.

How about the data set shown inFig. 4? In fact, the snapshot was takenduring open-loop, closed-throttle de-celeration when fuel was not being in-jected, so the O2 sensor PID makessense. It’s actually a substituted defaultvalue inserted whenever the vehicle isin closed-throttle decel mode. Whatabout the reported MAP value? Areading of 4.00 in./Hg shows very highengine vacuum, which jibes with thereported TPS PID. The fuel trim datais within the usually accepted range of0 ±10%. Good data can come in a vari-ety of formats.

Of course, waveform captures fromyour scope are often all that are neededto confirm a faulty MAF sensor. In ourshop, we’ve found that a snap-throttleMAF test for Ford products should al-ways produce a peak voltage of at least3.8 volts DC. The snap-throttle test is

32 July 2006


Fig. 5 Fig. 6

Hot-wire and hot-film MAF sen-sors calculate airflow based onmonitoring the current re-

quired to maintain a constant tem-perature in the sensing element.When dirt accumulates, the addi-tional surface area allows greaterheat dissipation at low airflow rates.The dirt, however, also functions asan insulator, with an overall net re-sistance to heat transfer at very highairflow rates.

At idle and under relatively lowflow/load rate conditions where themajority of operation may takeplace, the surface area effect usual-ly predominates, causing a rich con-dition with fuel trim correctionsusually in the range of �10% to�5%. At sustained high flow/loadrates, the insulative effect usuallytakes over, causing a lean mixtureneeding fuel trim corrections ashigh as +30%.

Worse still is a complex case of

“mass confusion” that may arise un-der hard acceleration when long-term negative fuel trim corrections,learned in closed-loop under low-flow-rate conditions, are appliedprecisely when positive fuel trim cor-rections would be more appropriate.So, for example, when the systemgoes to open-loop during hard accel-eration where the MAF is alreadyunderreporting airflow by up to30%, the PCM may subtract an addi-tional 10% to 15% (LTFT) from thenormal fuel delivery calculation,leaving the system as much as 45%leaner than desired!

In midrange operation, the twoeffects (surface area and insulativeproperties) may roughly cancel eachout, with fuel trims being more orless normal. Additionally, the exactchemistry and configuration of dirtbuildups can vary, changing the bal-ance of power between the surfacearea and insulative effects.

How Contamination Affects Hot-Wire &Hot-Film MAF Sensors

performed the same way asfor ignition analysis. The ideais not to race the engine, butsimply to open the throttleabruptly to allow a momen-tary surge of maximum air-flow as the intake manifoldgets suddenly filled with air.It’s critical that the throttle beopened (and closed) as quick-ly as possible during this test.

The waveform in Fig. 5 onpage 32 is from a known-goodMAF sensor. Note the peakvoltage of 3.8 volts. The rapidrise and fall after the throttlewas first opened is normaland reflects the initial gulp ofair hitting the intake manifoldwalls and suddenly reaching maximumdensity, greatly reducing subsequentflow. The exact shape of the waveformmay vary from model to model, basedon intake manifold and air duct(snorkel) design.

What’s the relationship betweenMAF and engine speed? As Fig. 6shows, rpm and airflow rate track oneanother closely under the moderate ac-celeration conditions during which thisscreen capture was taken. The similarityof the shapes of the two traces shown

here suggests, but does not prove, thatthe MAF sensor is functioning well un-der these conditions. If the airflow re-port was consistently increased or de-creased by the same factor, say 10% oreven 50%, the shape of its graph wouldremain the same.

Consider the additional plots pre-sented in Fig. 7 above. Does the extradata shed any light on the MAF sensor’saccuracy? Or is this just an example oftoo much information?

Since short-term and long-term fuel

trims remain within singledigits throughout, we can bereasonably sure that the MAFsensor is functioning correctly.Do we really benefit fromlooking at the O2 sensor datahere? We could probably doalmost as well without it, sincewe have both STFT andLTFT, but the O2 trace (blue)serves as an additional cross-check on the validity of thefuel trim calculations. Moreimportantly, the O2 sensortrace proves both that an ap-propriately rich mixture wasobtained on hard accelerationand that applied fuel trimcorrections were effective

throughout the captured data set.I said at the outset that hard failures

were relatively rare, but they do occurfrom time to time, and I owe it to you todiscuss this type of failure as well as in-termittent failures. Open-circuited orshort-circuited MAF sensors usually seta trouble code, most frequently P0102or P0103 (low input and high input, re-spectively). P0100 is a nonspecific MAFsensor circuit fault, while P0104 indi-cates an intermittent circuit failure.Checking scan data is a vital first steptoward successful diagnosis of any ofthese codes. On pre-OBD II vehiclesespecially, unplugging a faulty MAFsensor will often restore a minimum de-gree of driveability as the PCM revertsto TPS, rpm and/or MAP as fuel deter-minants. Certain mid-’80s GM vehicleswere notorious for intermittent MAFsensor failures. These usually could beeasily recreated by lightly tapping with asmall screwdriver on the MAF sensorhousing at idle. A noticeable stumbleoccurring with each tap clinches thecondemnation (Fig. 8, page 36).

Of course, backprobing the MAFsensor connector for voltage drops atboth the power and ground terminalsKOER is a required step before any fi-nal condemnation. The coincidence ofVBATT and MAF both showing 0.0volts cannot be ignored. Neither shouldthe mouse nest in the MAF, nor thegnawed wires throughout the enginecompartment.

Why is this a hard diagnosis? Conta-

34 July 2006


Fig. 7

Most MAF sensor failures re-sult from contamination.Sometimes the dirt is visible,

but more often it’s not. Technicianshave tried a variety of cleaners, withmixed success. Many use an aerosolbrake/electrical parts cleaner, wait-ing until the MAF sensor is cold. AFord trainer in my area swears bythe most popular consumer glasscleaner. Several top technicians re-port good results from steam clean-ing, while others prefer a spray in-duction cleaner.

The vast majority of technicianswarn that the MAF sensor may bedamaged by any type of cleaningwhere the electrical connector is notheld upright. This is particularly truewhere strong chemicals are used, asthey may pool and work their way

into the delicate electronic circuitry. To avoid future contamination, be

wary of oiled air filters or any thatappear likely to shed lint. Poor seal-ing of air filter housings may con-tribute to contamination. Neverspray an ill-fitting air filter with a sili-cone lubricant or sealer; such spraysare likely to render the MAF sensorinaccurate. If an engine produces ex-cessive blowby gases, these may con-taminate the MAF sensor, as well. Besure any specified filter breather ele-ment is installed. If none is specified,but oil accumulates in the air intakehousing, the MAF sensor or associat-ed intake ducts, be sure to investi-gate and remedy the cause to pre-vent repeat failures. Be sure to checkmanufacturers’ TSBs, the iATNarchives and other sources as well.

Keeping It Clean

minated MAF sensors oftenoverreport airflow at idle (re-sulting in a rich condition andnegative fuel trim corrections)while underreporting airflowunder load (resulting in a leancondition and positive fueltrim corrections).

This double whammy makesdiagnosis more difficult for anumber of reasons: First, manytechnicians incorrectly elimi-nate the MAF sensor as a po-tential culprit because they ex-pect it to show the same bias(either over- or underreport-ing) throughout its operating range. Sec-ond, a lack of a direct MAF fault DTC(such as P0100) is often mistaken tomean that the MAF sensor must begood. Third, the symptoms mimic(among other possibilities) those of a ve-hicle suffering from low fuel pump out-put coupled with slightly leaking injec-tors or an overly active canister purgesystem. Even sluggish, contaminated or

biased oxygen sensors may cause similarsymptoms. Without appropriate testing,it’s hard to distinguish—just by driv-ing—among certain ignition or knocksensor faults and MAF sensor malfunc-tions. Additionally, since MAF sensorsare somewhat pricey, many techniciansare afraid to condemn them, fearing ei-ther the customer’s or the boss’ wrath iftheir diagnosis is not borne out. Perhaps

the biggest obstacle is lack of acomprehensive database ofknown-good waveforms, volt-ages and scan data againstwhich to compare the suspect.

My own data set featuresknown-good scan data andscope captures made KOEO,at idle and on snap-throttle. Ingeneral, these three data pointsshould be sufficient to identifya faulty MAF sensor even be-fore it sets a fuel trim code.

A bad Bosch hot-wire MAFsensor may be the result of afailed burn-off circuit. Don’t

simply replace the sensor; make surethe burn-off is functional. (The purposeof the burn-off is to clean the hot-wireof contaminants after each trip.) Burn-off is usually a key OFF function afterengine operation exceeding 2000 rpm.Burn-off circuit faults may be in thePCM or a relay. The hot-wire shouldglow visibly red during burn-off.

So what can we conclude from allthis? A broad and seemingly unrelatedor even contradictory range of fuel sys-tem-related driveability complaints mayarise from MAF sensor performancefaults. Fuel trim data showing excessivecorrections from base programmingcasts strong suspicion on MAF sensorperformance issues. After recording allDTCs and freeze frame data, many ex-perienced techs recommend unplug-ging a suspect MAF sensor to see if ba-sic driveability is improved. Scopetraces at idle and on snap-throttle accel-eration help verify MAF sensor guilt orinnocence.

As usual, a library of known-goodscan data and waveforms is invaluable.The Min/Max voltage feature on yourDMM may not be fast enough to catchactual peak voltage on a snap-throttletest, but is usually sufficient for verify-ing performance of frequency-generat-ing (digital) MAF sensors. If your scopeis capable of pulse-width triggering, us-ing that function will provide exact cap-tures of digital MAF sensors in snap-throttle testing.

36 July 2006


Fig. 8


Circle # 27

The Greek root gen- un-der lies many words incommon parlance—ge -neric is one of them.Your medical insuranceprovider and your phar-

macist both know that when it comes toprescriptions, generic equivalents cansave us all big money, with identical re-sults. In the last few years, ge neric has,for complex reasons, become a pejora-tive term, often used to convey the ideaof something of lesser quality than a so-called name-brand alternative.

Yet, for diagnostic purposes, thegeneric datastream sometimes offers abetter window into powertrain manage-ment operating conditions than eventhe name-brand “enhanced” or “manu-facturer-specific” interface can. In fact,even though I own several much morepowerful and expensive scan tools, Iroutinely use the generic interface re-siding on the cheapest of the bunch asmy go-to choice for initial code retrievaland data analysis.

This particular machine, an agingAuto Xray EZ6000, offers no bidirec-tional controls above code clearing, buthas the signal virtues of speed and avery high overall connectivity rate. It al-so quickly compiles a printable reportwhich includes current operating PIDs,DTCs (including pending codes) andfreeze frame data, all of which are obvi-ously useful. Individual monitor com-pletion status requires a separate query,as do both Mode $05 (oxygen sensortest results) and Mode $06 (monitorself-test results) data. Its nice graphingprogram makes data analysis easy aftera road test, and I’m actually happy thatyou cannot both read and record datasimultaneously, as the trees in myneighborhood view that particular be-havioral combination as an excuse tojump out in front of you.

One of the primary benefits of the

20 July 2014

BY SAM BELL

A resourceful diagnostician knows

complicated and expensive equipment

isn’t always needed. Tools with a narrower

focus, combined with an enlightened

approach, allow him to get the job done.

BY SAM BELL

A resourceful diagnostician knows

complicated and expensive equipment

isn’t always needed. Tools with a narrower

focus, combined with an enlightened

approach, allow him to get the job done.

DOING IT ALL WITHDOING IT ALL WITHP

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generic datastream stems from the re-quirement that it not display substitutedvalues. While many so-called enhancedinterfaces offer access to a greater num-ber of data PIDs, some of these may re-flect substituted values not based oncurrent operating data. For example,most Chrysler products will substitute areasonable guess for the actual intakemanifold vacuum value when the MAPsensor is unplugged. If you look in theenhanced datastream, you’ll see thatvalue varying quite believably as you revthe engine or drive the vehicle. If youlook at MAP_Volts, however, you’ll seea fixed value reflecting reference volt-age (Vref) for the sensor circuit. Buthow often do you actually look at thatPID instead of the vacuum reading?

While substituted values are prohibit-ed in the generic datastream, calculatedvalues are not. Thus, for example, anECT PID of �40°F reflects the calcu-lated temperature of an open ECT sen-sor circuit. In such cases, Toyota, for ex-ample, has for many years, then substi-tuted a value of 176°F in its enhanceddatastream, but not in the generic data.In our unplugged Chrysler MAP sensorexample above, using a generic inter-face, you’d see an unmoving value ofsomething in the neighborhood of255kPa or higher, corresponding to aboost pressure of about 25 psi above at-mospheric.

As a technical consultant to our stateEPA, several times a year I encountervehicles which have failed our OBD IIplug & play state emissions test for aMIL-on condition with one or more cur-rent DTCs that simply do not appear inthe “enhanced” interface, but which arereadily retrieved using a generic hook-up. I’m afraid I can’t shed a lot of lighton why this would occur since, clearly, itshould not. Thus far, I have not encoun-tered this issue in any 2008 or later vehi-cles. There seem to be a few makes

21July 2014

GENERIC DATASTREAMGENERIC DATASTREAM

which are more prone to this problem,but my data set is too sparse to be cer-tain of any meaningful correlation. Forthe moment, suffice it to say that thestate’s testing interface is also a genericone, and, apparently, there are instancesin which a DTC may set but not be re-trieved via even the factory scan tool. Onthese occasions, only a generic interfacewill work. As the saying has it, truth isstranger than fiction.

An additional advantage of using thegeneric datastream becomes apparentwhen you’re working on a vehicle forwhich your scan tool doesn’t provide anenhanced interface. Don’t laugh; I’vehad students call me up to ask what todo because they didn’t have a scan toolthat offered, say, a Saab or Daihatsu op-tion. A gentle reminder that they couldat least start in the generic interfaceusually nets an embarrassed oops! Be-cause the generic interface contains thedata most critical to engine operations(see the starred items in the “GenericPIDs” list on page 24), it’s normally suf-ficient to rule in or rule out a particulararea of concern such as fuel delivery, forexample, early in the diagnostic process.While you might well prefer to workwith a dealer-equivalent scan tool in al-most all cases, in the real world you maynot be able to justify buying a tool withlimited utility vis-à-vis your regular cus-tomer base.

Let’s take a look at what the genericinterface typically offers these days (seethe screen captures on this page). TheJ1979 SAE standard specifically defines128 generic data PIDs, but not all man-ufacturers use or support all of them.Some, such as Mode $01, PID$6F (cur-rent turbocharger compressor inletpressure), are highly specialized andwon’t apply to most current-productionvehicles, while others, such as PID $06(engine RPM), are pretty universal. Atypical PID list of current values (Mode$01) or of freeze frame values (Mode$02) would include some or all of the 74items listed in the generic PIDs list.Your scan tool may use slightly differentacronyms or abbreviations to identifyvarious data items.

Most of the PIDs in the list are prob-ably familiar to you, but a few may haveyou scratching your head. As you see,

starting with a model year 2005 phase-in, several new parameters have beenadded to the original generic data list.These include both commanded and ac-tual fuel-rail pressure, EGR commandand EGR error calculation, commandedpurge percentage, commanded equiva-lence ratio and a host of others, includ-ing many diesel-specific PIDs.

In-use counters may also indicatehow many times each of the various on-board monitors has run to completionsince the codes were last cleared. Thelist on page 24 includes most of thegeneric PIDs currently in widespreaduse. However, since not all manufactur-ers support all PIDs, and since theirchoices may vary by model, engineand/or equipment, the list given hererepresents only a portion of the PIDs

potentially supported. Additionally,manufacturers are free to establish anddefine supplemental modes and PIDswhich may or may not be accessible viaa generic interface. All ECUs with au-thority or control over emissions-relatedissues, however, must be accessible viathe generic interface.

From our generic PIDs list, I want tofocus on commanded equivalence ratiosfirst. In essence, this is the PCM’s wayof reporting how rich or lean a mixtureit’s commanding. The PID is presentedin a lambda format, with 1.0 indicating astoichiometric (ideal) air/fuel ratio.Larger numbers indicate more air—acommand to run at a leaner air/fuel ra-tio—while numbers less than 1.0 indi-cate a correspondingly richer mixture.If you have a gas analyzer capable of

DOING IT ALL WITH GENERIC DATASTREAM

These two screen shots show the 60 lines of generic data available from aknown-good 2014 Mazda CX-5, as captured via a Snap-on SOLUS Ultra scan tool.

22 July 2014

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displaying lambda, it should coincideextremely well with the CommandedEquivalence PID.

As with all fuel trim-related issues, it’s

best to check this PID at idle, at about1200 rpm and at about 2500 rpm. Ifyour actual tailpipe measurements don’tcoincide with the PID, be sure to check

for any exhaust leaks first. If there arenone, you’ll have to check for factorsthat could account for the discrepancy,such as fuel pressure faults, vacuum


24 July 2014

The five starred (★) criticalPIDs in the list below are

the most influential inputs. Vir-tually all the others functionmerely to fine-tune (trim) thebasic spark and fuel (basemap) commands mapped outin response to these PIDs. Thecause of any fuel trim correc-tions (STFT, LTFT) beyond therange of approximately �5%must be investigated.Standards Compliance - suchas OBD II (Federal), OBD II(CARB), EOBD (Europe), etc.MIL - malfunction indicatorlamp status (off/on)MON_STAT - monitor comple-tion status since codes clearedDTC_CNT - number of con-firmed emissions-related DTCsavailable for display★RPM - revolutions per min -ute: also, engine crankshaft (oreccentric shaft) speed, sourcedfrom the CKP★IAT - intake air temperature★ECT - engine coolant tem-perature★MAP and/or ★MAF - mani-fold absolute pressure or massairflow, respectively★TPS or ★TP - throttle positionsensor, usually given as calcu-lated percentage; see absoluteTPS belowCALC_LOAD - calculated, basedon current airflow, as percent-age of peak airflow at sea lev-el at current rpm, with correc-tion for current BAROLOOP - status: closed, closedwith fault, open due to insuffi-cient temperature, open dueto high load or decel fuel cut,open due to system faultSTFT_x (per bank) - short-term fuel trim; the percent-age of fuel added to or sub-tracted from the base fuelschedule (for speed, load,temperature, etc.) in order toachieve stoichiometry as de-termined by the relevantair/fuel or O2 sensor

LTFT_x (per bank) - long-termfuel trimVSS - vehicle speed sensorHO2SBxSy - heated oxygensensor, Bank x, Sensor y, suchas B1S2 for a bank 1 down-stream sensorIGN_ADV - ignition timing,measured in crankshaft de-greesSAS or SEC_AIR - commandedsecondary air status off/on; mayinclude information such as at-mosphere, upstream or down-stream of converter, command-ed on for diagnostic purposesRUN_TIME - seconds since lastengine start; some manufac-turers stop the count at 255secondsDISTANCE TRAVELED WITHMIL ON – in miles or kmFRP - fuel rail pressure relativeto intake manifold pressureFRP_G - fuel rail pressure,gauge readingO2Sx_WR_lambda(x) - widerange air/fuel sensor, bank x,equivalence ratio (0-1.999) orvoltage (0-7.999)EGR - commanded EGR per-centageEGR_ERR - deviation of sensedor calculated position fromcommanded position, percentPURGE - commanded percent-ageFUEL_LVL - fuel level input per-centage; can provide especiallyinvaluable information in freezeframe diagnostics of misfirecodes set under “ran-out-of-gas” conditions; unfortunately,not universally implementedWARMUPS - number of warm-ups since codes cleared; awarm-up is an ECT increase ofat least 40°F in which the ECTreaches at least 160°FDIST SINCE CLR - distancesince codes clearedEVAP_PRESS - evaporative sys-tem pressure

BARO - absolute atmosphericpressure (varies with altitudeand weather)O2Sx_WR_lambda(x) - equiv-alence ratio or current - widerange air/fuel sensor , position x,equivalence ratio (0-1.999) orcurrent (-128mA to +127.99mA)CAT_TEMP BxSy - catalyst tem-perature by bank and position(may be wildly unreliable)MON_STAT - monitor status,current tripCONT_MOD_V - control mod-ule voltage; usually measuredon the B+ input for the Keep-Alive-Memory (KAM) but maybe measured on a switched ig-nition input lineABS_LOAD - absolute load,percentage, 0-25,700% REL_TPS - relative throttle po-sition percentageAMB_AT or AMB_TEMP - am-bient air temperature; whereused, usually measured infront of the radiator, while IATor MAT (manifold air tempera-ture) are usually collected inthe intake ductwork, or insidethe throttle body or intakemanifold, respectivelyABS_TPx - absolute throttleposition, percentage, sensor Bor CAPP_x - accelerator pedal posi-tion sensors D-FTP_CMD - commanded throt-tle actuator percentageMIL_TIM - time run with MILon, minutesFUEL_TYP - fuel typeETOH_PCT or ETH_PCT -ethanol fuel %ABS_EVAP - absolute evapsystem vapor pressure, 0-327.675kPaEVAP_P or EVAP_PRESS - evapsystem vapor pressure (gauge),from -32,767 to +32,768PaSTFTHO2BxS2 – short-termsecondary (postcatalyst) oxy-gen sensor trim by bank

LTFTHO2BxS2 - long-term sec-ondary oxygen sensor trim bybankHY_BATT_PCT - hybrid batterypack remaining life, percent-ageE_OIL_T or ENG_OIL_TEMP -engine oil temperatureINJ_TIM - fuel injection timing,in crankshaft degrees from�210° BTDC to �302° ATDCFUEL_RAT - engine fuel rate involume per unit time—e.g.,liters per hour, gallons perminute, etc.TRQ_DEM - driver’s demandengine, percent torqueTRQ_PCT - actual engine, per-cent torqueREF_TRQ - engine referencetorque in Nm (0 to 65,535)TRQ_A-E - engine percenttorque data at A=idle; B, C, D,E = defined pointsAFC - commanded diesel in-take airflow control and rela-tive intake airflow positionEGR_TEMP - exhaust gas recir-culation temperatureCOMP_IN_PRESS - turbo -charger compressor inlet pres-sureBOOST - boost pressure con-trolVGT – variable-geometry turbocontrolWAST_GAT - wastegate con-trolEXH_PRESS - exhaust pressureTURB_RPM - turbocharger rpmTURB_TEMP - turbochargertemperatureCACT - charge air cooler tem-peratureEGTx - exhaust gas tempera-ture, by bankDPF - diesel particulate filterDPF_T - diesel particulate filtertemperatureNOX - NOX sensorMAN_TEMP - manifold sur-face temperatureNOX_RGNT - NOX reagent sys-temPMS - particulate matter sensor

Generic PIDs

leaks or a biased oxygen or air/fuel sen-sor. If you observe a close correlationwith lambda, you’ll be able to use thisPID with confidence in lieu of actuallambda readings while conducting addi-tional tests.

In general, you should expect thisPID to read very close to 1.00 at idle inclosed-loop operation with conventionaloxygen sensors in the upstream posi-tions. (Wide-range air/fuel [WRAF] ra-tio sensors may target alternate valuesunder various driving conditions, typi-cally targeting a leaner mix under light-throttle cruise, for example. Additional-ly, vehicles using gasoline direct injec-tion [GDI] may deviate from stoichiom-etry even at idle or under light-throttlecruise conditions.) Keep in mind thatthe name says a lot: This PID reportsthe command, not necessarily the effectof the command.

Once in a blue moon you may findthat commanded equivalence ratioseems to travel exactly opposite fromlambda, so that a Com_Eq_Rat of .95corresponds to an actual lambda value of1.05, for instance. After the one instancein which I’ve encountered this, I eventu-ally learned to think of the PID value asa deviation from 1.00, then move exactlythat far in the opposite direction. (Anunfortunate computer crash led to the

loss of my notes from that vehicle, and Ican no longer remember even whichforeign nameplate make it was, muchless the year, model and engine. What Ido remember is that it sure threw mefor a loop! I also remember recheckingthis at the time with another scan toolwith the same result, so I suspect that itwas simply the result of a mistranslationsomewhere along the way, and not a toolglitch per se.)

One more note on the commandedequivalence ratios PID: You’ll find it inuse for diesels as well. Stoichiometricconditions for gasoline engines result inan air/fuel ratio of approximately 14.7:1.The advent of oxygenated fuels has ac-customed us to seeing lambda valuesshowing slightly lean, up to as high as1.04 in some cases, with no apparentfault. Since fuel blends vary both region-ally and seasonally, normal values foryour area may differ. With diesels, theratio is closer to 14.5:1, with propanerunning best at 15.7:1 and natural gasworking out to about 17.2:1. If you’re us-ing your gas analyzer on a vehicle burn-ing one of these fuels, you’ll have to re-set your lambda calculations accordingly.

Most gas analyzers with a computerplotting interface readily accommodatemultiple fuel types, usually from thesetup menu. In the case of flex-fuel


26 July 2014

Value Description0 .........Not available1 .........Gasoline2 .........Methanol3 .........Ethanol4 .........Diesel5...........LPG (liquid propane gas)6...........CNG (compressed natural gas)7 .........Propane8 .........Electric9..........Bifuel running gasoline10..........Bifuel running methanol11 ........Bifuel running ethanol12 ........Bifuel running LPG13 ........Bifuel running CNG14.........Bifuel running propane15 .........Bifuel running electricity16.........Bifuel running electric

and combustion engine17 ........Hybrid gasoline18 ........Hybrid ethanol19 ........Hybrid diesel20 ........Hybrid electric21.........Hybrid running electric

and combustion engine22 ........Hybrid regenerative23 ........Bifuel running diesel

Any other value is reserved by ISO/SAE.There are currently no definitions for

flexible-fuel vehicles.

Fuel Type Table:Mode $01, PID $51

OBD - on-board diagnostics.OBD II - second-generation OBD, asspecified by SAE J1979.EOBD - Euro-specification OBD;slightly different from SAE-spec.JOBD - Japanese-specification OBD;slightly different from SAE-spec.DTC - diagnostic trouble code; P-codes refer to powertrain manage-ment faults; U-codes flag communi-cation network errors; B-codes relateto faults in body system manage-ment; C-codes are chassis systembased.PDTC - Permanent DTC; one thatcannot be cleared directly via scantool command; such codes will self-clear after the affected monitorshave successfully run to completionwith no further faults. PDTCs arewritten into a section of nonvolatilememory, so they persist even if the

battery is disconnected and all capac-itors are discharged.PID - parameter identification; a val-ue found in current or freeze framedata; may indicate a sensor reading,calculated value or command status.In a nongeneric (enhanced) interface,may indicate a substituted value.$- or -$ - prefix or suffix indicatingthat an alphanumeric string is hexa-decimal (presented in base 16.) TheJ1979 specifications which establishthe OBD II protocol are written usinghexadecimal notation throughout.Datastream - a set of PID values, DTCs,test results and/or PDTCs; the displayof such data on or via a scan tool.Freeze frame - a set of PID values in-dicating then-current data writteninto the PCM’s memory when a DTC

sets, similar to an aircraft flightrecorder. Note: Freeze frame data iserased when codes are cleared; besure to read and record before clear-ing DTCs.CAN - controller area network; also,communication via the same.Monitor - one or more self-tests exe-cuted by the OBD system to deter-mine whether a specific subsystem isfunctioning within normal limits.Monitor status changes to incom-plete or “not done” when DTCs arecleared, and returns to complete or“done” once all relevant self-testshave been run. A monitor statusshowing completion is not a guaran-tee of a successful repair unless thereare no codes and no pending codes,and unless the vehicle has been oper-ated under conditions similar tothose under which a previous faulthad occurred (see freeze frame).

Glossary

cars, check the ETOH_PCT PID tohelp your analyzer figure out the cor-rect stoichiometric ratio. Once you’vemade the proper selection, you canwork from lambda without bothering toknow or remember the exact stoichio-metric ratio involved.

I was certainly glad to see the appear-

ance of purge data in the generic list, asknowing the commanded purge statuscan assist in diagnosing several types ofdriveability faults above and beyondevap leaks and malfunctions. Remem-ber, however, that this PID reflects onlythe current commanded state, not nec-essarily what’s actually happening.

The PIDs for EGR Command andEGR Error are likewise helpful. De-pending on the interface you use, how-ever, EGR_Error may be reported“backwards,” with 100% indicating thatcommand and position are in completeagreement and 0% indicating that oneshows wide-open while the othershows shut. (I’ve seen this on numer-ous Hondas, where a 99.5% “error” ac-tually meant that the valve was closedas commanded.) As usual, a few min-utes checking known-good vehiclescan help avoid many wasted hourshunting problems that aren’t reallythere.

Other new PIDs inform us of themileage since the last time the codeswere cleared as well as the distancedriven since the MIL first illuminatedfor any current codes. Both of thesepieces of information can be useful, es-pecially if yours is not the first shop tolook at a particular problem. In the caseof intermittent faults, they can also helpgive you a better idea of just how fre-quently the issue does arise.

Beyond PIDSPotentially both more helpful and moreproblematic are the new PermanentDTCs found in mode $0A. These can-not be cleared directly via a scan tool,but will be self-erased once the corre-sponding monitors have successfullyrun to completion. Attempts to circum-vent plug & play emissions tests by sim-ply clearing codes without fixing theunderlying causes led to the develop-ment of these Permanent DTCs. Whilethere are times when I would ratherjust “kill the MIL,” the PDTCs makeme take the extra time to more fully ed-ucate my customers and to verify theefficacy of my repairs, often by resort-ing to Mode $06 data analysis.

The key thing to remember whenworking with Mode $06 data is that it’sentirely up to the OEM to define allTID$, CID$, MID$, etc. These defini-tions can vary by year, engine, modeland/or equipment even within thesame OEM division, so be sure to veri-fy the accuracy of any informationyou’re using to interpret this data be-fore you get yourself in trouble. Alsoremember that many manufacturers


28 July 2014

Circle #16

populate their Mode $06 datastreamwith “placeholder” values after codesare cleared and until affected monitorshave run to completion. This is a strongargument for waiting as long as possiblebefore clearing codes.

Probably 90% of the MIL-on com-plaints we see in my shop are resolvedusing “just” a generic scanner, coupled,of course, with a few decades of experi-ence! Nevertheless, since a genericscan interface can take you only so far,there are certainly other times whenwe break out one of our more sophisti-cated scan tools with bidirectional func-tionality, access to additional PIDs,guided diagnostics, etc.

Especially in an older vehicle, thegeneric communications data rate(baud speed) may also seem slow by to-day’s standards. After an initial scan, thislimitation can often be overcome by se-lecting a relatively small number ofPIDs relevant to the problem at hand.All vehicles since 2008 support CANcommunications even in the generic in-terface. The effective data transfer rates

here are plenty quick enough for almostany practical purpose.

Since OBD II generic standards donot apply beyond P-codes (and some U-codes), any full-service shop needs oneor more scanners to deal with B-, C-and most U-code issues. Remember,though, that many OEMs illuminateTRAC, VSC and/or ABS lights in re-sponse to any P-code. This is nearly uni-versally true in the case of drive-by-wire(electronic throttle body) applications,but may be found in many other in-stances as well. In all such cases, youmust resolve the P-code issue first, be-fore worrying about any of these side-effects codes. If you have an appropri-ate interface, once you’ve killed theMIL, clear those extra codes as well, sothe next tech doesn’t find them still inmemory if and when a legitimate B- orC-code ever does set.

The bottom line is that there are sev-eral potentially important advantages tousing a generic scan interface for initialcode retrieval and data analysis, so don’tbe afraid to get your feet wet! Since the

generic datastream focuses on the mostimportant inputs and commands, wherethe bulk of problems occur, and sinceall PID values reflect their associatedsensor states without substitution,you’re less likely to be capsized by aflood of irrelevant data.

As always, checking known-good ve-hicles will help keep you on an evenkeel and familiarize you with what“good” looks like. While you may occa-sionally wind up switching over to anenhanced interface, you’ll likely findthat routinely starting in generic using afast and inexpensive basic scanner re-sults in much greater efficiency.Whether your shop is large or small,this practice also lets you avoid exces-sive wear and tear on the more expen-sive and advanced scanners and keepsthem free for those longer-term diag-nostic challenges where their enhancedfeatures are actually needed.

29July 2014

This article can be found online atwww.motormagazine.com.

Circle #17

10 February 2015

The commanded equivalence(EQ) ratio parameter (PID) isrequired in the generic data-stream on all passenger vehi-cles since 2008 (see the EQ toair/fuel ratio [AFR] matrix on

the next page and the SAE definition in thebox on page 13). An EQ of 1 equals 14.7:1AFR. This PID should reflect the com-manded air/fuel ratio. That being said,there are no Bank1 and Bank2 EQ ratioPIDs, and can’t the EQ or AFR be differ-ent for each bank? Are the two averaged?Yipes! How is this going to work on avehicle?

This test was performed on a 2010 ToyotaTacoma with a 4.0L engine. It’s importantto note that this may not be representativeof other vehicles; this is one test. Also, thistest is not intended to be critical of any im-plementation of the EQ PID. I think theproblem is in the original definition of this

PID—there is no differentiation for Bank1and Bank2.

Now let’s get into the layout of the screencapture below, from the 2010 Tacoma:

In the top chart, rpm is in red (the scaleon the left-hand side) and vehicle speed isin green (the scale on the right-hand side).

In the second chart, air/fuel ratio sensorsBank1 and Bank2 are in milliamps, and bothare scaled on the left-hand side.

In the third chart, the air/fuel ratio sen-sors Bank1 and Bank2 are in volts, and bothare scaled on the left-hand side.

In the fourth chart. postcatalyst O2 Sen-sors Bank1 and Bank2 are in volts, and bothare scaled on the left-hand side.

In the last chart, the commanded EQ ra-tio is scaled on the left-hand side.

All data to the left of the dotted line inthe screen capture is a baseline test driveending with a long idle period prior to intro-ducing a skew in B1S1 AFR sensor (the ver-

New PIDs provide additional information that can be included in

your diagnostic efforts. But before it can be used, you must under-

stand how it was obtained and what it’s intended to represent.

Driveability Corner

[email protected]

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tical dotted line). The solid greenline is the point of measurement forthe reading in the small boxes tothe right of the parameter name onthe chart. Finally, I’ve drawn a solidfine black line horizontally in theEQ chart to show EQ equals 1.

Data AnalysisRemember that the air/fuel ratiosensors (charts 2 and 3) read highwhen lean and low when rich—theopposite of the O2 sensors that are

low when lean and high when rich.The high (lean) spikes in the AFRsensor data reflect deceleration fu-el-cut enleanment. Note the Bank1and Bank2 AFR sensors followingeach other closely in the baselinedata.I put in the AFR milliamp and

voltage data to demonstrate the tiny

amount of amperage used and theconversion to volts scaling. Note thepostcatalyst oxygen sensors also fol-lowing each other reasonably close-

ly. It’s noteworthy that at 105,000miles, the rear O2 sensors don’t goabove .8V and lay flat on zero forsome period of time. Perhaps these

11February 2015

Circle #17

Circle #18

Circle #19

Circle #20

continued on page 13


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sensors are showing some age.Note that the EQ PID in the

baseline data period is pretty activewhen the truck is being driven.Look at the EQ relative to the top

chart of rpm and mph to get an ideaof the changing load. Notice thatthe fuel-cut events that are reflect-ed well in the AFR, and O2 sensor

13February 2015

Driveability Corner

SAE Definition:Commanded EQ Ratio

Fuel systems that utilize convention-al oxygen sensors shall display thecommanded open-loop equivalenceratio while the fuel control system isin open loop. EQ_RAT shall indicate1.0 while in closed-loop fuel.Fuel systems that utilize wide-

range/linear oxygen sensors shalldisplay the commanded equivalenceratio in both open-loop and closed-loop operation.To obtain the actual A/F ratio be-

ing commanded, multiply the stoi-chiometric A/F ratio by the equiva-lence ratio. For example, for gaso-line, stoichiometric is 14.64:1 ratio. Ifthe fuel control system was com-manding a .95 EQ_RAT, the com-manded A/F ratio to the enginewould be 14.64 x 0.95 = 13.9 A/Fratio.

data are not well represented in theEQ data. The EQ ratio data looksalmost backwards when comparedto the rich periods on the AFR’s(low) and the O2’s (high). The EQlooks like it’s going in the oppositedirection.Okay, now let’s look at the point

of defect. I skewed the B1S1 AFRsensor. You can see the immediateskew in the AFR sensor and O2 sen-sor data. The O2 sensor rails at thebottom (lean). The AFR B1S1 ini-tially skews down, recovers at idleand then skews down again underload. The AFR sensor is skewed tolook rich, a false signal I created.The fuel response is to react to leanthe “rich” mixture. The rear O2 sen-sor shows the enleanment and theEQ shows the command to lean.Is the EQ using just Bank1? Is it

an average of both banks? Rightnow I have more questions than an-swers. I’ll skew Bank2 next timeand see where it leads.

At 105,000miles, the rear O2

sensors don’tgo above .8Vand lay flat onzero for someperiod of time.


6 November 2014

Déjà Vu All Over AgainA 2007 Chevy Impala with a 3.5L engine cameinto our shop for the first time about a year anda half ago. The MIL was on, the engine had re-duced power and DTC P0121 (Throttle Posi-tion Sensor 1 Performance) was stored in thePCM memory. I removed, cleaned and re-mounted the throttle body, then flashed thePCM and inspected the wiring harness. TheDTC did not return, so the vehicle was re-turned to the customer. About six months later,the throttle body failed with the same DTCP0121 stored. At this time the throttle body wasreplaced with a remanufactured aftermarketpart. The wiring harness was also rerouted, asit did not appear to have enough slack betweenthe body and the engine.Fast-forward another six months or so and

the throttle body failed again. The symptoms

were the same (reduced power) and the sameDTC P0121 was stored in memory. I followedall of the recommended diagnostic procedures,then replaced the throttle body (a second time).We recently heard from the customer, and thevehicle is apparently experiencing the all-too-familiar reduced power symptoms and theCheck Engine light is on. Is it time to install anew OE part? I have not previously had anyproblem with reman parts purchased from thissupplier. Is there an underlying issue that isshortening the throttle body’s life span? I don’twant to throw any more of the customer’s mon-ey at this problem without finding an answer.

Jerry BurnsTrenton, NJ

Due to the large amount of time that haselapsed between each failure occurrence, we’dhave to consider this to be a very intermittent

KarlSeyfert

One definition of insanity is repeating the same action and expect-

ing a different outcome. After multiple replacements of the same

part, it may be saner to look elsewhere for the cause of the failure.

[email protected]

Trouble Shooter

Perhaps due to safety considerations, this 2007 Chevy Impala throttle-by-wire throttle housing has “nouser serviceable parts inside.” Any accumulated gunk can be removed from the area around the throttleblade, but familiar adjustments to components like the TP sensor are no longer possible.

Pho

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problem. Intermittents are certainlymore difficult, but not impossible, todiagnose.Perhaps the most helpful informa-

tion that could be used to solve thisproblem would be the freeze framedata. This would tell you the operatingconditions at or near the moment whenthe DTC P0121 was stored. Were thefreeze frame data parameters the same(or similar) each time the DTC wasstored? If they were, is it because morethan one throttle position sensor hasfailed in exactly the same way? Thisscenario is not impossible, but it seemsstatistically unlikely, unless a series offaulty parts were involved.In general terms, what do we know

about the possible causes of a P0121? Itbegins when the PCM detects a mal-function that’s causing an excessivelylow or high voltage signal to be sentfrom throttle position sensor to thePCM. This can be caused by a throttleposition sensor that has an internalfault. Since the sensor can’t be replacedseparately, this is probably why you’vebeen installing replacement throttlebody assemblies. The DTC can also becaused by a throttle position sensor har-ness that’s open or shorted. A poor orintermittent electrical connection inthe throttle position sensor circuitcould also be to blame. Lastly, andprobably the least likely, the PCM maybe experiencing intermittent failures.Because this is a throttle-by-wire

system, the PCM responds to prob-lems with its inputs by reducing enginepower. Under normal conditions, thePCM uses the TP sensor input to de-tect the actual position of the throttlevalve, as well as the opening and clos-ing speed of the throttle valve. If theTP sensor reports that the throttlevalve is closed, the PCM would usethis information to control other func-tions, such as fuel cut.If the PCM does not have accurate

information about how far open orclosed the throttle valve may be at agiven moment, it can’t accurately con-trol the opening and closing of thethrottle from that point on. The re-duced engine power allows the driverto (barely) limp the car into a servicefacility. This may be an inconvenience,

but should be considered safer than thepossibility of a runaway throttle.When the customer brings the vehi-

cle to your shop this time, make certainyou capture the freeze frame data be-fore making any changes to the PCMor its programming. When did theDTC store? What was happening atthe time? With the original throttlehousing still in place, make your bestattempt to duplicate these conditions.Monitor the relevant PIDs with yourscan tool. To open an even larger win-dow into this problem, attach a digitalstorage oscilloscope to the TP sensor’sdata lines. Watch the scope for any in-dication of signal abnormalities as youwork the throttle through its normalrange of movements. This may be atemperature-related failure, so it maybe necessary to drive the vehicle longenough to get everything under thehood good and warm.There are a few harness connections

between the TP sensor and the PCM.Examine each of them closely for anysigns of looseness, fretting or otherdamage. You mentioned that the har-ness appeared too tight between thethrottle housing and the body. Is it pos-sible that this is or was causing a har-ness connector to partially separate,causing the TP sensor signal to weakenor intermittently drop out? Once again,manipulating the harness while observ-ing the TP sensor signal on the scopemay allow you to capture an intermit-tent failure.Lastly, there’s your question about

the quality of the parts involved, andthe possible link to repeated failures. Inmy research, I found that throttle hous-ing failures are not unheard of on thesevehicles, so the original equipmentparts certainly are not unbreakable.Some techs have experienced problemswith remanufactured replacementparts, while others have not. Beforepointing the finger of blame at any re-placement part, be it original equip-ment or aftermarket, new or remanu-factured, I’d suggest you first make cer-tain you’ve eliminated all of the otherpossible problem causes. Installing an-other throttle housing without doing somight buy you some time, but déjàvu could still be a possibility.

8 November 2014

Trouble Shooter

Circle #6


4 October 2014

Ready to Buy a Cat?I am having a problem with a DTC P0420 that’spuzzling me. The vehicle is my 2009 Cadillac STS,which has about 85,000 miles on it. It’s equippedwith a 3.6L direct fuel injection V6 engine, a six-speed automatic transmission and AWD.

The freeze frame data indicates bank 1 was inopen loop when the P0420 was set. Bank 2 was inclosed loop. I cleared the code and it came backin about 300 miles. Once again, bank 1 was inopen loop when the DTC set. During subsequenttesting, both bank 1 and bank 2 went into closedloop within a few minutes after engine starts.

The B1S1 oxygen sensor switches normally,and responds to throttle (wide-open and closed).Even though it appeared to be functioning nor-mally, I replaced the B1S1 O2 sensor with an OEpart anyway, thinking it might be an intermittentO2 sensor. After that, I cleared the code, but itcame back again in about 700 miles.

I checked for intake leaks (with propane) andexhaust leaks (by plugging the tailpipe with a rag

and listening for a sound change). I was unable tofind any leaks. The BARO reading of 98KPaseems to be in agreement with where I live(Metro Detroit area).

I am trying to rule out other possible expla-nations for the P0420 before having the faith toreplace the cat. What puzzles me is why bank 1is in open loop while bank 2 is in closed loopwhen P0420 is set.

I searched for the most common OBD II codeson an automotive reference website. P0420 was attop of the list (with 13.2% of the total). P0430 wasnumber 10 with 3.2% of the total. I can’t think ofa reason why there are significantly more P0420sthan P0430s recorded. I think this is interesting.

I typically try to spend enough time on a di-agnosis until I am confident about making arecommendation to replace any parts. But thisP0420 with bank 1 in open loop puzzles me. Ihave included the freeze frame data that wasstored at the same time the DTC was set. Iwould appreciate your help.

Pusheng ChenNovi, MI

Any DTC that sets only every 300 to 700 miles isgoing to be tough to diagnose. And one that pointsto possible replacement of an expensive emissionscontrol component like a catalytic converter whenit does is going to be even tougher. So first let meapplaud your dedication. And second, thank youfor the foresight to save and include the freezeframe data with your note. This data may not pro-vide all of the information we need to reach a diag-nostic conclusion, but it should help to get uspointed in the right direction.

P0420 is a very popular DTC—or unpopular,depending on how you look at it. It indicates thatthe PCM has determined that the catalytic con-verter is performing below an establishedthreshold. OBD II’s number one mission is tokeep vehicle emissions as low as possible, and itcan’t do that without a properly functioning cat-alytic converter (or converters, in some cases).OBD II keeps a close eye on converter perform-

KarlSeyfert

Deciding to replace an expensive emissions control component

requires confidence in the accuracy of your diagnosis. Is the

decision tougher or easier when you’reworking on your own car?

[email protected]

Trouble Shooter

Freeze frame data is perhaps the most useful information available when at-tempting to determine the cause an OBD II diagnostic trouble code. This datais collected at the moment the DTC is set and is the next best thing to beingthere when it happens. What can the data shown here tell us about theP0420 that was set on the 2009 Cadillac STS when it was collected?

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ance, and when performance drops be-low a prescribed level, the PCMwill seta DTC. Some might argue that per-formance levels are set too tightly, mak-ing it all too easy for a converter to failan OBD II monitor.Many vehicles are equipped with 4-

cylinder engines, which typically havejust a single catalytic converter, or onelarge and one small cat, coupled with aset of pre- and postconverter oxygensensors situated at either end of themain cat. Your STS is equipped withtwo of everything because it’s a V6 withseparate emissions equipment for eachbank. P0430 points to a bank 2 catalyticconverter that’s operating below an es-tablished threshold. Many vehiclesdon’t have this second converter, whichI believe explains why P0430 is so muchfurther down on the “hit parade” ofDTCs, when compared to the chart-topping P0420. If all vehicles had two ofeverything, the two catalyst efficiencyDTCs would probably be more evenlyranked in terms of occurrence.The PCM doesn’t have a five-gas ex-

haust analyzer probe stuck up thetailpipe of your STS, so how does itmake the determination that the con-verter is functioning below the perform-ance threshold? The PCM runs a cata-lyst monitor test only when certain driv-ing conditions have been met. The en-gine and converter must be at operatingtemperature, and the engine may beidling or running under light load at lowspeed. Your freeze frame data indicatesthe STS’s engine speed was 1228 rpm.The fuel system should also be inclosed-loop fuel control (this is key).There must not be any other unfulfilledcriteria or previously stored DTCs thatwould keep the catalyst efficiency moni-tor from running.Once the PCM has determined that

all preconditions have been met, it tem-porarily forces the air/fuel mixture rich,to deplete any stored oxygen in the con-verter. Then the PCM temporarilyforces the air/fuel mixture lean to deter-mine how long it takes for the converterto react and for the downstream oxygensensor to change its switching activity. Ifthe converter takes too long to resumefunctioning (indicated by postconverteroxygen sensor activity), it means the cat-

alyst is not working efficiently enough tomaintain the vehicle’s emissions levelswithin prescribed limits. OBD II willthen fail the converter, set a DTC P0420and turn on the Check Engine light.I believe your vehicle is setting a

DTC P0420 only every 300 to 700 milesbecause that’s how long it takes for all ofthe preconditions to be met, and for thePCM to run the catalyst efficiency mon-itor. Alternately, the monitor may berunning more frequently, and failingonly once every 300 to 700 miles.The key piece of information con-

tained in the freeze frame data is the in-dication that bank 1 was in open loop atthe time the freeze frame data wasstored. On the face of it, this makes nosense, as the catalyst efficiency monitorshould never have run in the first placewith half of the fuel system still in openloop. Achieving closed loop is one of thefirst preconditions the fuel systemwould have to satisfy before the PCMwould even consider running the cata-lyst efficiency monitor.We know that freeze frame data is

stored at the moment the PCM decidesto flag a DTC. So in this case the datawas probably collected a certain periodof time after the PCM attempted to runthe catalyst efficiency monitor. The fuelsystem had to be in closed loop whenthe monitor began to run, but some-thing happened after that, and it was nolonger in closed loop when the freezeframe data was stored. This is a hypoth-esis, as we don’t know how quickly thePCM updates the freeze frame datawe’re now using for our diagnosis.I’d suggest you look for a component

that’s capable of intermittently kickingthe fuel system out of closed loop. Thismay be happening at other times, be-sides when the PCM is attempting torun the catalyst efficiency monitor. Be-sides the pre- and postcatalyst oxygensensors, most of the other input sensorshave their own OBD II DTCs thatshould give you an indication of a prob-lem. But it may be too intermittent totrigger a DTC and the only way youmay be able to identify it is by monitor-ing a limited set of PIDs, waiting for theglitch to reveal itself. It can’t hide forev-er, and you’ve already shown that youhave the patience to wait.

7October 2014

Trouble Shooter


Gary Stamberger – Training Director Magnaflow Exhaust Products

In the first part of this OBD II Code Diagnosis series I stated that we would discuss the principles of OBD II codes and breakdown each character that defines them. For a generic discussion of OBD I’ll refer you to TB-80016 and 80017. We archive all of our bulletins and they can be found on our website at www.maganaflow.com. Look for Tech Bulletins under Tech Support. For this series I would like to stay on a more specific path. In our first two parts we took a very common Ford EGR code and broke down the diagnosis. I chose this code not only for its commonality but also because this EGR system uses several components, each one playing a major role in the vehicles ability to reduce NOx. Although the PCM has the ability to set several different and distinct codes for each component (9 generic and 10 specific) the interrelation of the components cannot be ignored. As we saw in our example, one of the possible causes for the P0401 code was mechanical and had nothing to do with the malfunction of any one component. Another common issue in Code Diagnostics sometimes overlooked is that of retrieving codes in both OBD II Generic and Enhanced or Manufacture Specific mode. Depending on the tool being used, the enhanced option may not be available (i.e. Code Reader only). Using generic mode requires less input therefore is faster and in most cases will get the technician to where he wants to be. The downside is that it is a generic code and therefore in many cases the repair information will not be specific to that vehicle. The obvious upside then to using Enhanced Mode, is that the diagnostic information will be specific to that vehicle or at least that manufacturer. The description and operation will give you a better idea of what the PCM is looking for and the subsequent testing should lead you to the proper diagnosis the first time. Example: 2005 Altima, 2.5L with an illuminated MIL. The OBD II code was P0140, O2 Circuit B1S2 No Activity Detected. A quick glance at the data stream showed that under the proper test conditions the sensor displayed activity. At this point we might determine that it is an intermittent problem, clear the code and send the customer on their way. However a look at Enhanced codes revealed a P1147, O2 B1S2 Maximum Voltage not Obtained. A closer look at data stream showed that the sensor was not reaching a specific maximum voltage of .78v. This specific information was not available when processing the P0140 code. The key to any diagnostic situation is to always follow a pattern for each problem we face and code diagnostics is no different. Yes… each manufacture has common problems and knowing where to find that information is valuable but sometimes even the “silver bullet” can be a dud! Whether it is a no start, misfire, won’t idle, MIL illuminated or any number of issues, having a plan is by far the best plan. “Shot Gun” diagnosis will on occasion allow us to hit the illusive homerun but more often than not we spend a whole day repairing a component only to go home with that empty feeling in our stomachs, knowing the same problem will reoccur in the morning. Diagnostics is an art and getting good at it can be a great confidence booster, however these vehicles are changing constantly and there is no time to rest. As I say when closing all my classes:

THE RULES ARE ALWAYS CHANGING TECHNOLOGY KEEPS MOVING FORWARD

EDUCATION IS A CONTINUAL PROCESS Cleaning up the environment…one converter at a time Gary

OBD II Code Diagnosis Part III

Bulletin TB-80035 September, 2011

Gary Stamberger – Training Director Magnaflow Exhaust Products As promised from last month, more on OBD. Refer to our Website, Magnaflow.com for archived Bulletins. (http://www.magnaflow.com/07techtips/techbulletins.asp) Data Stream Referred to as Current Data or Live Data, this information is available to the technician using a Scan Tool. The number of PIDS (Parameter Identification) available at any given time will depend on a couple of different factors. The particular vehicle (Manufacturer) involved will have the greatest influence on the amount of data available. Followed by the type of Scan Tool used and whether you are viewing the data on the Global OBD II side or Manufacture Specific, aka Enhanced Mode. (Figure 1) Most Scan Tools will have options for viewing the data in different formats such as digital or graphing mode. Graphing can be particularly useful when looking at Oxygen Sensor activity. (Figure 2) The data available will consist of inputs and outputs, calculated values and system status information. Viewing data and becoming proficient at recognizing problem areas is one of the skills we spoke of in last months Bulletin (TB-80016). Part of any training on a particular tool is the repetitive process of using it over and over until you begin to recognize when certain data doesn’t look right. This process will then lead you toward a problem area where further testing will reveal the fault. You can not recognize bad data until you have looked at enough good data. One item to be aware of is the practice of substituting good data values for suspect ones. Due to something called Adaptive Strategy, when the PCM suspects that a particular input may not be reporting accurately, it will substitute a known good value for that sensor and run the vehicle on learned values. This will only show up in Enhanced Mode as Global OBD II will always display actual values. This should not deter you from viewing in Enhanced Mode. It has always been my practice to look at codes and data in both modes.

FIGURE 1 FIGURE 2 Freeze Frame Freeze frame is a “snap shot” of data taken when a code is set. This can be very valuable information as it allows the technician an opportunity to duplicate the conditions under which the trouble code was recorded. The number of freeze frame events recorded and viewable by the technician will again depend on the vehicle and scan tool being used. Early systems could only store one batch of information, if more than one code was recorded we would typically only be able to view the Freeze Frame for the last code set. Changes in both OBD and Scan Tool technology have allowed us to have multiple sets of information available for multiple codes set. One exception is that of Misfire. Misfire codes and subsequent data take precedent and will overwrite any previous freeze data stored. Be aware that all freeze frame information is lost when codes are cleared.

On Board Diagnostics Part II

Bulletin TB-80017 December, 2009

FIGURE 3 FIGURE 4 Courtesy Toyota Monitors Monitors, also referred to as Readiness Indicators are considered the single most comprehensive change that came with OBD II. CARB and the EPA recognized that a vehicle started polluting long before the PCM recognized a fault, set a code and illuminated the MIL. Early OBD systems did not have the capability to recognize degradation of components or systems. Today’s OBD II system is designed to recognize when a vehicle could potentially exceed its designed emission standard by a factor of 1.5. It does this through a series of system Monitors. During normal operation the PCM will conduct certain tests to gauge the operational health of a particular system or component. The Monitors operate in two categories, Continuous and Non-Continuous. As you can probably guess the Continuous Monitors run, well, continuously. They are Misfire, Fuel System and Comprehensive Component. Non-Continuous consist of Catalyst, Evaporative, Oxygen Sensor, Oxygen Sensor Heater, EGR Monitor and more. These require a very specific Drive Cycle (Figure 4) that will meet all the criteria necessary for a complete test. Scan Tools will have a Monitor Status screen that indicates if the Monitors have run to completion. (Figure 3) Next to each component or system it will indicate “Ready” or “Not Ready”, “Complete” or “Incomplete”. If the vehicle is not equipped with a certain system the screen will indicate “Not Supported” or “Not Available”. When one or more indicators read Not Ready or Incomplete, it is an indication that codes have been cleared recently, either with a scan tool or loss of power to the PCM such as battery disconnect. If there is no history of either of these events occurring this is an indication of the PCM intermittently loosing power or it is rebooting which could be an internal problem. It is commonly known that the Catalyst and Evaporative System Monitors are the hardest to run to completion. Many states have moved to an OBD system test for Emission Testing in place of tail pipe testing for vehicles 1996 and newer. California is considering this transition as we move into 2010 (No date has been set for implementation). The test includes checking for proper location of DLC (Data Link Connector), bulb check of MIL, no MIL when vehicle is running, no codes in system and all the Monitors have run to completion. Monitors are a key component because they are a direct indication of whether the OBD system had been tampered with prior to Inspection. The USEPA and CARB authorities have generally found that OBD II systems are more effective in detecting emission-related malfunctions on in-use vehicles compared to existing Inspection and Maintenance (I/M) tailpipe testing procedures. Current Smog Check data indicates that vehicles are more likely to fail an OBD II-based inspection than the required tailpipe emissions test. With the reduced testing times (10 mins. for OBD vs. 20 mins. for tail pipe) and cost savings in equipment it’s not beyond the realm of possibility that states currently having none or minimal Inspection Programs may consider adopting an OBD Emissions Testing program. These programs have proven to create a healthy environment and also a healthy bottom line for repair shops. Cleaning up the environment…one converter at a time Gary

Gary Stamberger – Training Director Car-Sound/Magnaflow Performance Exhaust

This month we take the discussion of Oxygen Sensors to yet another level. In recent discussions we talked about the role these sensors played in closed loop fuel control. What exactly does that mean, “Closed loop fuel control”, and what role does it play in maintaining a good working converter? When a vehicle is started cold there is a warm up period which is referred to as, “Open loop”. It’s during this time period that the engine is polluting the most. Consequently, getting to closed loop fuel control is a top priority. The PCM has an internal clock that restarts on each start-up and it knows, based mainly on temperature, how long before all components are operating and it is ready to enter closed loop. To this end, many elements have been added to the systems. Oxygen sensors have built in heaters to speed the warm up process. The PCM can detect when the engine is taking too long to come up to temperature and will set a code P0125, “Insufficient temperature for closed loop fuel control” which typically means the thermostat is stuck open. Once the conditions are met and the PCM gains fuel control the goal then becomes maintaining it. The oxygen sensor is referred to as a, “Voltage Generator” and reports the content of oxygen in the exhaust stream to the PCM ranging between 100mv (Millivolts) and 900mv. When the oxygen content is high, (Voltage is low, near 100mv) the PCM sees this as a lean condition and its response is to add fuel. When the sensor reports back that there is little oxygen in the exhaust stream (high voltage, near 900mv), a rich condition is sensed and the PCM pulls fuel away. A technician can monitor this data on a scan tool as, “Short Term Fuel Trim” or STFT. A positive percentage indicates the computer is adding fuel while a negative number says it is taking fuel away. If the PCM is in fuel control, monitoring the direct relationship between O2 and STFT scan data will confirm it.

The next step then is to look at Long Term Fuel Trim (LTFT) percentages. These numbers give us a history of what the PCM has been doing with fuel trim over the long haul. As with STFT, positive percentages tell us the tendency is to be adding fuel (compensating for a lean condition) while negative numbers indicate the PCM is pulling fuel back, (Overcoming a rich condition). If either of these conditions exists for a prolonged period of time and the LTFT percentages exceed the PCM’s parameters a fuel trim code will set (P0170-P0175) and Check Engine light illuminated. The example below shows us that although the PCM appears to be in fuel control there is evidence that it has been adding fuel over time.

Our concern when looking at fuel trim is what it may be telling us about engine efficiency and whether the computer has been compensating for other fuel related problems. If the engine has been over-fueling the question is…WHY? A leaking fuel injector, fuel pressure regulator, lazy O2, or bad Mass Air Flow (MAF) would be some of the considerations. The same issue exists if it’s too lean. Here an air leak, clogged injectors or fuel filter, or miscalculated air flow could be the cause. Any Fuel Trim condition that persists will eventually take its toll on the catalytic converter and must be addressed by the repair technician before installing a new one. Cleaning up the environment…one converter at a time Gary

INTERPRETING FUEL TRIM DATA

Bulletin TB-80010 May, 2009

November 2006 Premier Issue

Practical uses of Mode $06, by Phil Fournier Page 1 of 1

Practical uses of Mode $06 Round out your diagnostic skills

By Phil Fournier



As I worked to get a handle on the presentation of Mode 6 at a technician’s level, I was reminded of a slide in one of my PowerPoint presentations that announces “the lab scope allows the technician a look inside the manufacturer’s electronic strategy.” I’m going to make a similar statement here regarding the use of Mode 6. Mode 6 gives the technician a look inside the manufacturer’s strategy. Sometimes it’s a blurry look, and sometimes it’s a look of limited usefulness, but taking the look is worth the trouble. It will make the technician who takes the trouble a better-rounded diagnostician, even if he/she only uses it for a few selected items. What It Is First off we have to cover some basics for those still uncertain of what mode 6 is or isn’t. Mode 6 is part of the SAE standards that defined what kind of data would be available to technicians through the OBD2 interface. Simply put, it is the brains behind the operation of the OBD2 monitors of various emission control systems. In theory, it covers what we know as the non-continuous monitors, those usually run by the OBD2 system one per trip if the conditions are right. By now, we all know that those include Fuel Evap, Catalyst, O2 sensor, O2 sensor heater, EGR, and so forth. But the cool thing about the information available in some mode 6 data is that it breaks down the monitor into its various parts, sometimes giving us useful information that cannot be seen as well through looking at live data stream or looking at stored trouble codes. I’m going to start off by suggesting that if you are serious about learning the benefits of Mode 6, invest a few bucks in a scan tool capable of doing the interpretation for you. If you don’t know what I mean by that, it means you are not currently using Mode 6. The first time I stumbled across Mode 6 data was while randomly pushing buttons on my scan tool and looking at stuff. I rapidly backed out of the screen due to what looked to me to be completely useless information, filled with $ signs, things called TID’s and CID’s, plus letters and numbers mixed. And so it is unless you have a way to interpret the data. This is because Mode 6 was written in Hexadecimal code (Base 16 instead of Base 10) and not particularly designed with the technician in mind. But never mind, there are plenty of things we have learned to use that fall into this same category. Hex to Key Because this article is designed to be useful information, I don’t want to get bogged down in a boring discussion of Hexadecimal numbers and Base 16. I will just suggest though that you can use your free Windows calculator to convert the letters and numbers into pure numbers. If you use “Scientific” from the “View” menu, you will get a choice of the “Decimal” calculator or the “Hexadecimal” calculator. Entering the letter and number combination into the Hexadecimal screen and then clicking the button for Decimal will convert the number to a readable number. Unfortunately, that number will still do you



little good unless you know what it means via a conversion key. Part of the reason that this article is based on Ford vehicles is because the Ford information is available for free at www.motorcraftservice.com. Choose “OBD2 Theory and Operation” from the menu on the left of the screen, then scroll down and pick the Adobe Acrobat file to open. Once the file is open you can go to the monitor that you want to look at. Misfire Counts But leaving all that behind, there is one area of Mode 6 on a Ford which you can start using immediately, as long as you have some scan tool that will display Mode 6 (and not all of them do; I’ll include a list later in the article of assorted scan tools and how to find Mode 6 in them.) I refer to misfire counts, which are not contained in Ford’s regular Enhanced data stream on the majority of scan tools. Test ID $51 in 1996-98 vehicles and Test ID $53 from 1997 and on in others will display misfire counts WITH THE CORRESPONDING CYLINDER shown as the Component ID ($01 through $0A). Note that $0A is Hex code for the number 10 and indicates cylinder number 10. If you have less than ten cylinders, you can ignore the data in any of the CID’s above your number of cylinders because it is bogus. But the beauty of this data is that at last you can identify which cylinder has misfire counts in it, especially when you have NO CODE. The reason for this can be seen in the data. Look at the figure below, captured off a 1999 Merc Marquis with no codes, but a complaint of a misfire under certain load conditions. Note that I am using the AutoEnginuity PC based Scan tool that interprets the data for us but you can most likely use your own scanner in a similar fashion.

Notice that all cylinders have zeros in the misfire counts column except cylinder #3. The counts are low, nowhere near the threshold required to set a code. But this information was invaluable on a coil-on-plug engine that I had no way to connect to in looking for a misfire under load condition. After inspecting the plug boot and spark plug for any sign of arching, and finding none, all I had to do to verify the coil was failing was to swap it with another cylinder and take the car for a second road test, after clearing codes. (Note: Though there were no codes, a code clear procedure removes the data from Mode 6 misfire monitors; this is not necessarily true of all data in Mode 6 though.) Finding the misfire has followed the coil to a different cylinder, the coil can be replaced with perfect confidence in a proper repair.

http://www.motorcraftservice.com



It is a bit ironic that Mode 6 data on a Ford contains misfire counts, since SAE J1979 defined Mode 6 as non-continuous monitor information and Mode 7 as continuous. Never mind, we’ll take the data for its usefulness, even if it is in the wrong place. But this little snafu is symptomatic of Mode 6 data in general. Not all things are as they might be expected to be, which makes some technicians throw up their hands and conclude that the moving target is not worth the trouble. But let’s carry on and see what other use we might get out of it. Cat Stats I know many technicians who sweat over the replacement of a catalyst because of a code P0420/P0430. When factory cats cost in excess of $800 each, it is small wonder that they worry about a misdiagnosis. But how about if we could record the Mode 6 data, clear the code, and then drive the car to see what resets in the Mode 6 monitor? Let’s see what our 1999 Grand Marquis with 108k miles on it shows for Mode 6 data on its catalyst monitor.

This cat monitor was run after the coil was replaced, and we can safely conclude by looking at the data that this catalyst is still in good condition. The switch ratio of the bank 1 cat (the side of the misfiring coil) is actually a bit better than that of bank 2. And both cats are well under the limit described here as .842, which according to the Ford website reflects a limit of the percentage of switching of the rear O2 sensor as opposed to the front one. Notice what happens to the data when we clear codes:

It didn’t go to zero, did it? Instead, some random number got put in the box, a number that looks like a near-failure of .749. But what’s the likelihood that both cats would measure the exact same switch ratio? Just about zero, but this illustrates the need to not be careless in your treatment of Mode 6 information. The graphic below (captured from a 1998 Ford Winstar) show what the data would like uninterpreted:

See Chart on following page!



You could do the math, converting the numbers from Hex to Decimal, and then multiplying by the conversion factor listed on the website of .0156. But why bother? It is easy to see that the two catalysts on this vehicle are well within the maximum limit. But what if you had a code P0420, cleared it, then drove the vehicle and saw the 10/11 test number at 19? You could be pretty comfortable at recommending the catalyst, and depending on where you saw the 10/21 parameter, you might be recommending a pair. On the following pages… Restricted EGR and scan tool tips



Choked off? Restricted EGR is one other place where Mode 6 on a Ford can come in handy. Here is data from a normal system (1999 Ford Crown Vic 4.6L with no problems):

Notice that the TID $45/$20 is the same as the DPFE voltage as long as the EGR valve is not stuck open. This can help a technician that may not have enhanced Ford data, as the generic data stream (Mode 1) does not list DPFE voltage as a parameter. Listed below is the data (same vehicle) where I disconnected the intake side hose on the inconveniently located DPFE (see photo ?):

Note that this set a pending code P0401(EGR flow insufficient) on a single road test. However, because we have no specs for TID 41/CID11 & 12, they do not help us like they are supposed to and the vehicle does not set the code P1405 like it is supposed to. But obviously something is detected. Next I reconnected the DPFE hose and installed a restriction in the EGR valve (see figure ?) to simulate a restricted EGR passage. I captured the following data:

I find it very interesting that TID $4A/$30 has barely passing numbers, but in order to achieve that much flow, the computer had to ratchet up the EVR duty cycle to 89%. However, in spite of repeated road tests and completed EGR monitors, this condition would not set even a pending code. Below is another capture with the EGR blocked completely:



Taken all together, we can see that Mode 6 stored data can help us nail down a restricted EGR passage that does not set a code, now that we know to look at TIDs $4A/$30 and $4B/30. You can probably do that on your scan tool without an interpreter, but you are going to have to do the Hex plus conversion math and use the Ford website. Conclusion In conclusion, Mode 6 is a barrel of data, some of it bogus and meaningless, and much of it powerful. I’m told that Honda recommends the use of Mode 6 data in the diagnosis of fuel evaporative problems. I’d recommend you dive into your scan tool and pick some TID’s and CID’s to figure out so you can get started on learning what to expect, particularly if you have the good fortune to work on a single car line. If you are a multicar line guy like me, try it out on Ford misfires to start with and see if you don’t find it to be a real time saver.

How to access Mode $06 in assorted scan tools (found under Generic or Global OBD II in every case):

MasterTech: Select “F5 System Test” then “F2 Other Results” (Note that results are displayed as “Pass/Fail”. To get the actual readings press “*, Help”) Snap On: (later than 2001 cartridge, earlier versions don’t have Mode 6): After communicating with vehicle select “Display Test Parameters/Results” then select “Non- Continuous Monitored systems (Mode $6). (MT2500; Solus, Modis similar) BDM: Select “Non-Continuous Monitor Test Results” NGS: Select Diag Monitoring Test Results AutoEnginuity: Select On Board Test Results tab OTC Genesis: Select “Special Tests” then “Component Parameters”

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