Description of On-Board Diagnostics

HARD CODE

Powertrain
A Diagnostic Trouble Code (DTC) that comes back within one cycle of the ignition key is a HARD code. This means that the problem is present when the PCM checks that circuit or function. Most procedures here verify if the trouble code is a hard code at the beginning of each test. When it is not a hard code, an "intermittent" test must be performed.

Codes that are for OBDII monitors will not set with just the ignition key on. Comparing these to non-emission codes, they will seem like an intermittent. These codes require a set of parameters to be performed (The DRB III pre-test screens will help with this for MONITOR codes), this is called a "TRIP". All OBD II DTCs will be set after two or in some cases one trip failures, and the MIL will be turned on. These codes require three successful, no failures, TRIPS to extinguish the MIL, followed by 40 warm-up cycles to erase the code. For further explanation of TRIPS, Pre-test screens, Warm-up cycles, and the use of the DRB III, refer to the On Board Diagnostic training booklet # 81-699-97094.

Transmission
Any Diagnostic Trouble Code (DTC) that is set whenever the system or component is monitored is a HARD code. This means that the problem is there every time the PCM/TCM checks that system or component. Some codes will set immediately at start up and others will require a road test under specific conditions. It must be determined if a code is repeatable (Hard) or intermittent before attempting transmission diagnosis.

ONE TRIP FAILURE
A One Trip Failure, when read from the PCM/TCM, is a hard OBDII/EURO III code that has not matured to the full 5 minutes. This DTC can take up to five minutes of problem identification before illuminating the MIL

INTERMITTENT CODE
A diagnostic trouble code that is not present every time the PCM/TCM checks the circuit or function is an INTERMITTENT code. Most intermittent codes are caused by wiring or connector problems. Intermittent conditions that come and go like this are the most difficult to diagnose; they must be looked for under specific conditions that cause them.

Intermittent transmission speed ratio codes are usually caused by intermittent hydraulic seal leakage in the clutch and/or accumulator circuits. Intermittent speed ratio codes can be set by intermittent speed sensor circuitry or by line noise being induced onto one or both of the speed sensor signal circuits.

The following procedures may assist you in identifying a possible intermittent problem:
- Visually inspect related wire harness connectors. Look for broken, bent, pushed out, or corroded terminals.
- Visually inspect the related harnesses. Look for chafed, pierced, or partially broken wire.
- Refer to any S.T.A.R. Hotline Newsletters or technical service bulletins that may apply.
- Use the DRB III data recorder or co-pilot.

STARTS SINCE SET COUNTER

Powertrain
This reset counter counts the number of times the vehicle has been started since codes were last set or erased. This counter will count up to 255 start counts. The number of starts helps determine when the trouble code actually happened. This is recorded by the PCM and can be viewed on the DRB III as STARTS since set. When there are no trouble codes stored in memory, the DRB III will display "NO TROUBLE CODES FOUND" and the reset counter will show "STARTS since set = XXX. OBD II vehicles will also display a DTC Specific or Global "Good Trip" counter which will indicate the number of "Good Trips" since the DTC was set. After 3 consecutive "Good Trips," the MIL is extinguished and the good trip counter is replaced by a "Warm Up Cycle" counter. 40 Warm-Up Cycles will erase the DTC and Freeze Frame information.

Transmission
The Starts Since Set counter counts the number of times the vehicle has started since the most recent DTC was set. The counter will count up to 255 starts. Note that this counter only applies to the last code set.

When there are no diagnostic trouble codes stored in memory, the DRB III will display "NO DTC's PRESENT" and the reset counter will show "STARTS SINCE CLEAR" = XXX.

The number of starts helps determine if the diagnostic trouble code is hard or intermittent.
- If the number of starts is less than 3, the code is usually a hard code.
- If the number of starts is greater than 3, it is considered an intermittent code. This means that the engine has been started most of the time without the code recurring.

TROUBLE CODE ERASURE
A Diagnostic trouble code will be cleared from TCM memory if it has not reset for 40 warm-up cycles.

A warm-up cycle is defined as "sufficient vehicle operation such that the coolant temperature has risen by at least 22 °C (40 °F) from engine starting and reaches a minimum temperature of 71 °C (160 °F).

The Malfunction Indicator Lamp (MIL) will turn off after 3 good trips or when the DTC's are cleared from the TCM.