DOING IT ALL WITH GENERIC DATASTREAM

While substituted values are prohibited in the generic datastream, calculated values are not. Thus, for example, an ECT PID of - 40°F reflects the calculated temperature of an open ECT sensor circuit. In such cases, Toyota, for example, has for many years, then substituted a value of 176°F in its enhanced datastream, but not in the generic data. In our unplugged Chrysler MAP sensor example above, using a generic interface, you'd see an unmoving value of something in the neighborhood of 255kPa or higher, corresponding to a boost pressure of about 25 psi above atmospheric.

As a technical consultant to our state EPA, several times a year I encounter vehicles which have failed our OBD II plug & play state emissions test for a MIL-on condition with one or more current DTCs that simply do not appear in the "enhanced" interface, but which are readily retrieved using a generic hookup. I'm afraid I can't shed a lot of light on why this would occur since, clearly, it should not. Thus far, I have not encountered this issue in any 2008 or later vehicles. There seem to be a few makes which are more prone to this problem, but my data set is too sparse to be certain of any meaningful correlation. For the moment, suffice it to say that the states testing interface is also a generic one, and, apparently, there are instances in which a DTC may set but not be retrieved via even the factory scan tool. On these occasions, only a generic interface will work. As the saying has it, truth is stranger than fiction.

An additional advantage of using the generic datastream becomes apparent when you're working on a vehicle for which your scan tool doesn't provide an enhanced interface. Don't laugh; I've had students call me up to ask what to do because they didn't have a scan tool that offered, say, a Saab or Daihatsu option. A gentle reminder that they could at least start in the generic interface usually nets an embarrassed oopsl Because the generic interface contains the data most critical to engine operations (see the starred items in the "Generic PIDs" list on page 24), it's normally sufficient to rule in or rule out a particular area of concern such as fuel delivery, for example, early in the diagnostic process. While you might well prefer to work with a dealer-equivalent scan tool in almost all cases, in the real world you may not be able to justify buying a tool with limited utility vis-à-vis your regular customer base.

Let's take a look at what the generic interface typically offers these days (see the screen captures on this page). The J1979 SAE standard specifically defines 128 generic data PIDs, but not all manufacturers use or support all of them. Some, such as Mode $01, PID$6F (current turbocharger compressor inlet pressure), are highly specialized and won't apply to most current-production vehicles, while others, such as PID $06 (engine RPM), are pretty universal. A typical PID list of current values (Mode $01) or of freeze frame values (Mode $02) would include some or all of the 74 items fisted in the generic PIDs list. Your scan tool may use slightly different acronyms or abbreviations to identify various data items.

Most of the PIDs in the list are probably familiar to you, but a few may have you scratching your head. As you see, starting with a model year 2005 phasein, several new parameters have been added to the original generic data fist. These include both commanded and actual fuel-rail pressure, EGR command and EGR error calculation, commanded purge percentage, commanded equivalence ratio and a host of others, including many diesel-specific PIDs.

In-use counters may also indicate how many times each of the various onboard monitors has run to completion since the codes were last cleared. The list on page 24 includes most of the generic PIDs currently in widespread use. However, since not all manufacturers support all PIDs, and since their choices may vary by model, engine and/or equipment, the fist given here represents only a portion of the PIDs potentially supported. Additionally, manufacturers are free to establish and define supplemental modes and PIDs which may or may not be accessible via a generic interface. All ECUs with authority or control over emissions-related issues, however, must be accessible via the generic interface.

From our generic PIDs list, I want to focus on commanded equivalence ratios first. In essence, this is the PCM's way of reporting how rich or lean a mixture it's commanding. The PID is presented in a lambda format, with 1.0 indicating a stoichiometric (ideal) air/fuel ratio. Larger numbers indicate more air-a command to run at a leaner air/fuel ratio-while numbers less than 1.0 indicate a correspondingly richer mixture. If you have a gas analyzer capable of displaying lambda, it should coincide extremely well with the Commanded Equivalence PID.

As with all fuel trim-related issues, it's best to check this PID at idle, at about 1200 rpm and at about 2500 rpm. If your actual tailpipe measurements don't coincide with the PID, be sure to check for any exhaust leaks first. If there are none, you'll have to check for factors that could account for the discrepancy, such as fuel pressure faults, vacuum leaks or a biased oxygen or air/fuel sensor. If you observe a close correlation with lambda, you'll be able to use this PID with confidence in lieu of actual lambda readings while conduofing additional tests.

In general, you should expect this PID to read very close to 1.00 at idle in closed-loop operation with conventional oxygen sensors in the upstream positions. (Wide-range air/fuel [WRAF] ratio sensors may target alternate values under various driving conditions, typically targeting a leaner mix under lightthrottle cruise, for example. Additionally, vehicles using gasoline direct injection [GDI] may deviate from stoichiometry even at idle or under light-throttle cruise conditions.) Keep in mind that the name says a lot: This PID reports the command, not necessarily the effect of the command.

Once in a blue moon you may find that commanded equivalence ratio seems to travel exactly opposite from lambda, so that a Com Eq Rat of .95 corresponds to an actual lambda value of 1.05, for instance. After the one instance in which I've encountered this, I eventually learned to think of the PID value as a deviation from 1.00, then move exactly that far in the opposite direction. (An unfortunate computer crash led to the loss of my notes from that vehicle, and I can no longer remember even which foreign nameplate make it was, much less the year, model and engine. What I do remember is that it sure threw me for a loop! I also remember rechecking this at the time with another scan tool with the same result, so I suspect that it was simply the result of a mistranslation somewhere along the way, and not a tool glitch per se.)

One more note on the commanded equivalence ratios PID: You'll find it in use for diesels as well. Stoichiometric conditions for gasoline engines result in an air/foel ratio of approximately 14.7:1. The advent of oxygenated fuels has accustomed us to seeing lambda values showing slightly lean, up to as high as 1.04 in some cases, with no apparent fault. Since fuel blends vary both regionally and seasonally, normal values for your area may differ. With diesels, the ratio is closer to 14.5:1, with propane running best at 15.7:1 and natural gas working out to about 17.2:1. If you're using your gas analyzer on a vehicle burning one of these fuels, you'll have to reset your lambda calculations accordingly.

Most gas analyzers with a computer plotting interface readily accommodate multiple fuel types, usually from the setup menu. In the case of flex-fuel cars, check the ETOH PCT PID to help your analyzer figure out the correct stoichiometric ratio. Once you've made the proper selection, you can work from lambda without bothering to know or remember the exact stoichiometric ratio involved.

I was certainly glad to see the appearance of purge data in the generic list, as knowing the commanded purge status can assist in diagnosing several types of driveability faults above and beyond evap leaks and malfunctions. Remember, however, that this PID reflects only the current commanded state, not necessarily what's actually happening.

The PIDs for EGR Command and EGR Error are likewise helpful. Depending on the interface you use, however, EGR Error may be reported "backwards," with 100% indicating that command and position are in complete agreement and 0% indicating that one shows wide-open while the other shows shut. (I've seen this on numerous Hondas, where a 99.5% "error" actually meant that the valve was closed as commanded.) As usual, a few minutes checking known-good vehicles can help avoid many wasted hours hunting problems that aren't really there.

Other new PIDs inform us of the mileage since the last time the codes were cleared as well as the distance driven since the MIL first illuminated for any current codes. Both of these pieces of information can be useful, especially if yours is not the first shop to look at a particular problem. In the case of intermittent faults, they can also help give you a better idea of just how frequently the issue does arise.

Beyond PIDS

Potentially both more helpful and more problematic are the new Permanent DTCs found in mode $0A. These cannot be cleared directly via a scan tool, but will be self-erased once the corresponding monitors have successfully run to completion. Attempts to circumvent plug & play emissions tests by simply clearing codes without fixing the underlying causes led to the development of these Permanent DTCs. While there are times when I would rather just "kill the MIL," the PDTCs make me take the extra time to more fully educate my customers and to verify the efficacy of my repairs, often by resorting to Mode $06 data analysis.

The key thing to remember when working with Mode $06 data is that it's entirely up to the OEM to define all TID$, CID$, MID$, etc. These definitions can vary by year, engine, model and/or equipment even within the same OEM envision, so be sure to verify the accuracy of any information you're using to interpret this data before you get yourself in trouble. Also remember that many manufacturers populate their Mode $06 datastream with "placeholder" values after codes are cleared and until affected monitors have run to completion. This is a strong argument for waiting as long as possible before clearing codes.

Probably 90% of the MIL-on complaints we see in my shop are resolved using "just" a generic scanner, coupled, of course, with a few decades of experience! Nevertheless, since a generic scan interface can take you only so far, there are certainly other times when we break out one of our more sophisticated scan tools with bidirectional functionality, access to additional PIDs, guided diagnostics, etc.

Especially in an older vehicle, the generic communications data rate (baud speed) may also seem slow by todays standards. After an initial scan, this limitation can often be overcome by selecting a relatively small number of PIDs relevant to the problem at hand. All vehicles since 2008 support CAN communications even in the generic interface. The effective data transfer rates here are plenty quick enough for almost any practical purpose.

Since OBD II generic standards do not apply beyond P-codes (and some Ucodes), any full-service shop needs one or more scanners to deal with B-, Cand most U-code issues. Remember, though, that many OEMs illuminate TRAC, VSC and/or ABS lights in response to any P-code. This is nearly universally true in the case of drive-by-wire (electronic throttle body) applications, but may be found in many other instances as well. In all such cases, you must resolve the P-code issue first, before worrying about any of these sideeffects codes. If you have an appropriate interface, once you've killed the MIL, clear those extra codes as well, so the next tech doesn't find them still in memory if and when a legitimate Bor C-code ever does set.

The bottom line is that there are several potentially important advantages to using a generic scan interface for initial code retrieval and data analysis, so don't be afraid to get your feet wet! Since the generic datastream focuses on the most important inputs and commands, where the bulk of problems occur, and since all PI D values reflect their associated sensor states without substitution, you're less likely to be capsized by a flood of irrelevant data.

As always, checking known-good vehicles will help keep you on an even keel and familiarize you with what "good" looks like. While you may occasionally wind up switching over to an enhanced interface, you'll likely find that routinely starting in generic using a fest and inexpensive basic scanner results in much greater efficiency. Whether your shop is large or small, this practice also lets you avoid excessive wear and tear on the more expensive and advanced scanners and keeps them free for those longer-term diagnostic challenges where their enhanced features are actually needed.

Generic PIDs

The five starred (?) critical PIDs in the list below are the most influential inputs. Virtually all the others function merely to fine-tune (trim) the basic spark and fuel (base map) commands mapped out in response to these PIDs. The cause of any fuel trim corrections (STFT, LTFT) beyond the range of approximately ±5% must be investigated.

Standards Compliance - such as OBD II (Federal), OBD II (CARB), EOBD (Europe), etc.

MIL - malfunction indicator lamp status (off/on)

MON_STAT - monitor completion status since codes cleared

DTC_CNT - number of confirmed emissions-related DTCs available for display

?RPM - revolutions per minute: also, engine cranlahaft (or eccentric shaft) speed, sourced from the CKP

?IATintake air temperature

?ECT - engine coolant temperature

?MAP and/or *MAF - manifold absolute pressure or mass airflow, respectively

?TPS or *TP - throttle position sensor, usually given as calculated percentage; see absolute TPS below

CALC_LOAD - calculated, based on current airflow, as percentage of peak airflow at sea level at current rpm, with correction for current BARO

LOOP - status: closed, closed with fault, open due to insufficient temperature, open due to high load or decel fuel cut, open due to system fault

STFT_x (per bank) - shortterm fuel trim; the percentage of fuel added to or subtracted from the base fuel schedule (for speed, load, temperature, etc.) in order to achieve stoichiometry as determined by the relevant air/fuel or 02 sensor

LTFT_x (per bank) - long-term fuel trim

VSS - vehicle speed sensor

H02SBxSy - heated oxygen sensor, Bank x, Sensor y, such as B1S2 for a bank 1 downstream sensor

IGN_ADV - ignition timing, measured in crankshaft degrees

SAS or SEC_AJR - commanded secondary air status off/on; may include information such as atmosphere, upstream or downstream of converter, commanded on for diagnostic purposes

RUNJÏME - seconds since last engine start; some manufacturers stop the count at 255 seconds

DISTANCE TRAVELED WITH MIL ON-in miles or km

FRP - fuel rail pressure relative to intake manifold pressure

FRP_G - fuel rail pressure, gauge reading

02Sx_WR_lambda(x) - wide range air/fuel sensor, bank x, equivalence ratio (0-1.999) or voltage (0-7.999)

EGR - commanded EGR percentage

EGR_ERR - deviation of sensed or calculated position from commanded position, percent

PURGE - commanded percentage

FUELJ.VL - fuel level input percentage; can provide especially invaluable information in freeze frame diagnostics of misfire codes set under "ran-out-ofgas" conditions; unfortunately, not universally implemented

WARMUPS - number of warmups since codes cleared; a warm-up is an ECT increase of at least 40°F in which the ECT reaches at least 160°F

DIST SINCE CLR - distance since codes cleared

EVAP_PRESS - evaporative system pressure

BARO - absolute atmospheric pressure (varies with altitude and weather)

02Sx_WR_lambda(x) - equivalence ratio or current - wide range air/fuel sensor, position x, equivalence ratio (0-1.999) or current (-128mA to +127.99mA)

CATTEMP BxSy - catalyst temperature by bank and position (may be wildly unreliable)

MON_STAT - monitor status, current trip

CONT_MOD_V - control module voltage; usually measured on the B+ input for the KeepAlive-Memory (KAM) but may be measured on a switched ignition input line

ABS_LOAD - absolute load, percentage, 0-25,700%

RELJPS - relative throttle position percentage

AMB_AT or AMB.TEMP - ambient air temperature; where used, usually measured in front of the radiator, while IAT or MAT (manifold air temperature) are usually collected in the intake ductwork, or inside the throttle body or intake manifold, respectively

ABS_TPx - absolute throttle position, percentage, sensor B orC

APP_x - accelerator pedal position sensors D-F

TP_CMD - commanded throttle actuator percentage

MILJÏM - time run with MIL on, minutes

FUELTYPfuel type

ETOH_PCT or ETH.PCT - ethanol fuel %

ABS_EVAP - absolute evap system vapor pressure, 0327.675kPa

EVAP_P or EVAP_PRESS - evap system vapor pressure (gauge), from-32,767 to+32,768Pa

STFTH02BxS2 - short-term secondary (postcatalyst) oxygen sensor trim by bank

LTFTH02BxS2 - long-term secondary oxygen sensor trim by bank

HY_BATT_PCT - hybrid battery pack remaining life, percentage

E_OIL_T or ENG_OIL_TEMP - engine oil temperature

INJ_TIM - fuel injection timing, in crankshaft degrees from -210° BTDC to +302° ATDC

FUEL_RATengine fuel rate in volume per unit time-e.g., liters per hour, gallons per minute, etc.

TRQ_DEM - driver's demand engine, percent torque

TRQ_PCT - actual engine, percent torque

REF_TRQ - engine reference torque in Nm (0 to 65,535)

TRQ_A-E - engine percent torque data at A=idle; B, C, D, E = defined points

AFC - commanded diesel intake airflow control and relative intake airflow position

EGR_TEMP - exhaust gas recirculation temperature

COMP_IN_PRESS - turbocharger compressor inlet pressure

BOOST - boost pressure control

VGT -variable-geometry turbo control

WAST_GAT - wastegate control

EXH_PRESS - exhaust pressure

TURB_RPM - turbocharger rpm

TURB_TEMP - turbocharger temperature

CACT - charge air cooler temperature

EGTx - exhaust gas temperature, by bank

DPF - diesel particulate filter

DPF_T - diesel particulate filter temperature

NOX - NOx sensor

MAN_TEMP - manifold surface temperature

NOX_RGNT - NOx reagent system

PMS - particulate matter sensor

Glossary

OBD - on-board diagnostics.

OBD II - second-generation OBD, as specified 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; Pcodes refer to powertrain management faults; U-codes flag communication network errors; B-codes relate to faults in body system management; C-codes are chassis system based.

PDTC - Permanent DTC; one that cannot be cleared directly via scan tool command; such codes will selfclear after the affected monitors have successfully run to completion with no further faults. PDTCs are written into a section of nonvolatile memory, so they persist even if the battery is disconnected and all capacitors are discharged.

PID - parameter identification; a value found in current or freeze frame data; may indicate a sensor reading, calculated value or command status. In a nongeneric (enhanced) interface, may indicate a substituted value.

$- or -$ - prefix or suffix indicating that an alphanumeric string is hexadecimal (presented in base 16.) The J1979 specifications which establish the OBD II protocol are written using hexadecimal notation throughout.

Datastream - a set of PID values, DTCs, test results and/or PDTCs; the display of such data on or via a scan tool.

Freeze frame - a set of PID values indicating then-current data written into the PCM's memory when a DTC sets, similar to an aircraft flight recorder. Note: Freeze frame data is erased when codes are cleared; be sure to read and record before clearing DTCs.

CAN - controller area network; also, communication via the same.

Monitor - one or more self-tests executed by the OBD system to determine whether a specific subsystem is functioning within normal limits. Monitor status changes to incomplete or "not done" when DTCs are cleared, and returns to complete or "done" once all relevant self-tests have been run. A monitor status showing completion is not a guarantee of a successful repair unless there are no codes and no pending codes, and unless the vehicle has been operated under conditions similar to those under which a previous fault had occurred (see freeze frame).

This article can be found online at www.motormagazine.com.