Hard Failures
Most hard failures cause MIL to illuminate and remain on until malfunction is repaired. If MIL illuminates and remains on (light may flash) during vehicle operation, cause of malfunction must be determined. See DIAGNOSTIC PROCEDURE . If a sensor fails, PCM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but it will most likely display degraded driveability.
Intermittent Failures
Intermittent failures cause MIL to flash, or illuminate and turn off about 10 seconds after intermittent fault goes away. Corresponding DTC, however, will be retained in PCM memory. If related fault does not reoccur within 40 engine starts, DTC will be erased from PCM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in appropriate TROUBLE SHOOTING - NO CODES article.
FREEZE FRAME DATA
Engine operating conditions are required to be captured and stored in PCM memory whenever MIL is illuminated. Data captured is called FREEZE FRAME data. Freeze frame data is similar to a single record of operating conditions. Whenever MIL is illuminated, corresponding record of operating conditions is recorded to freeze frame buffer.
Freeze frame data can only be overwritten with data associated with a misfire or fuel trim malfunction. Misfire data takes priority over all other data because of its related part-damaging effects (oxygen sensors, catalytic converters, etc.). Freeze frame data will not be erased unless associated DTC is cleared from PCM memory.
During an intermittent fault, MIL may illuminate, and then turn off after 3 trips. However, corresponding DTC will be stored in PCM memory. Check for an intermittent malfunction when an unexpected DTC appears. If an intermittent DTC can not be resolved by following test procedures related to DTC, or if no DTC is set, see INTERMITTENTS in appropriate TROUBLE SHOOTING - NO CODES article.
RETRIEVING DTCS
DTCs are retrieved using a Tech 1 or 2 scan tool, or other OBD-II compatible scan tool connected to OBD-II 16-pin Data Link Connector (DLC). (Scheme 12) DLC is located below instrument panel, on driver's side of vehicle.
Scheme 12
CLEARING DTCS
| CAUTION | To prevent damage to PCM when clearing DTCs, ignition switch must be in LOCK position when disconnecting and connecting power to PCM. |
Note. Do not clear DTCs until directed by diagnostic procedure. When DTCs are cleared, freeze frame data which could be used for intermittent fault diagnosis, will also be erased from memory.
To clear DTCs using scan tool, select CLEAR DTCs or CLEAR INFORMATION function. Follow scan tool manufacturer's instructions.
If a scan tool is not available, DTCs can be cleared by disconnecting one of the following power sources for at least 30 seconds
- Power source to PCM (such as battery pigtail, PCM connectors, fuse, etc.).
- Negative battery cable (loss of radio presets and/or clock settings will result).
PCM will clear stored DTC if fault that caused DTC to set is not detected for 40 consecutive warm-up cycles.
SUMMARY
If no hard fault codes are present, driveability symptoms exist or intermittent DTC(s) exist, proceed to appropriate TROUBLE SHOOTING - NO CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.
CIRCUIT TESTS
Note. Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM before proceeding with CIRCUIT TESTS.