On-Board Diagnostics: Notes

NOTE: Before performing any testing procedures, check for any related technical service bulletins.

The MIL may also be referred to as the CHECK ENGINE or SERVICE ENGINE SOON light. When ignition is first turned on, MIL should come on for a short period to verify bulb and circuit operation, and then go off. The MIL is located on instrument cluster, left of tachometer. If MIL fails to come on when ignition is first turned on, or remains on steady with no DTCs stored in PCM, diagnose MIL malfunction. See TEST NTC-1: NO TROUBLE CODE TEST MENU under DIAGNOSTIC TESTS and perform appropriate test.

The Powertrain Control Module (PCM) monitors several different engine control system circuits. If PCM detects a malfunction which affects vehicle emissions, a Diagnostic Trouble Code (DTC) will be stored in PCM the next time engine is started if malfunction still exists on all conditions except engine misfire or fuel rich/lean condition. On engine misfire or fuel system rich/lean condition, DTC will be stored when malfunction occurs. When DTC is stored, PCM enters limp-in mode. In limp-in mode, PCM substitutes values for the failed component to continue engine operation, but a loss of good driveability may result. If PCM detects active engine misfire, MIL will flash or remain on steady and DTC will be stored. If problem exists which does not affect vehicle emissions, a DTC will be stored in PCM but MIL will not be turned on.

The PCM will turn off MIL when malfunction is not detected during 3 consecutive engine start-to-stop trips on all applications except an engine misfire or fuel rich/lean condition. On engine misfire or fuel rich/lean conditions, PCM will turn off MIL when malfunction is not detected when condition such as engine speed, engine coolant temperature etc. are similar to those when malfunction was first recorded.

If the PCM does not detect the recent malfunction during 51 engine starts for non-emission related DTCs or 40 drive cycles for engine misfire or fuel rich/lean condition DTCs, the DTC will be cleared from the PCM.

The PCM also records and stores engine operating conditions when the malfunction occurred. This information is referred to as freeze frame data. Freeze frame data may be obtained using scan tool, and used for simulating tests for trouble shooting. Following freeze frame data is available.

Calculating Load Value
DTC Data Recording
Engine Coolant Temperature
Engine Speed
Fuel Control Condition Such As Open Or Closed Loop
Long Term Fuel Compensation (Fuel Trim)
MAP Sensor Vacuum
Short Term Fuel Compensation (Fuel Trim)
Vehicle Speed

DTCs may be retrieved for system diagnosis. See RETRIEVING DIAGNOSTIC TROUBLE CODES under SELF-DIAGNOSTIC SYSTEM. System malfunctions are identified as either hard failures or intermittent failures. See HARD FAILURES or INTERMITTENT FAILURES .