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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

INTERPRETING GENERIC SCAN DATA

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

INTERPRETING GENERIC SCAN DATA

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

INTERPRETING GENERIC SCAN DATA

Fig. 5

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