Malfunction Indicator Lamp: Description and Operation

OPERATION
The Powertrain Control Module (PCM) performs an on-board Diagnostic (OBD) check of the EGR system on all California vehicles. The diagnostic system uses the electronic EGR Transducer (EET) for the system tests.

The OBD check activates only during selected engine/driving conditions. When the conditions are met, the PCM energizes the EET solenoid to disable the EGR. the PCM checks for a change in the oxygen sensor signal. If the air/fuel mixture goes lean, the PCM will attempt to enrich the mixture. The PCM registers a Diagnostic Trouble Code (DTC) if the EGR system has failed or degraded. After registering a DTC, the PCM turns the Malfunction Indicator Lamp (MIL) ON. The MIL indicates the need for immediate service.

If a malfunction is indicated by the MIL and a DTC for the EGR system was set, check for proper operation of the EGR system. If the EGR system tests properly, check the system using the DRB II scan tool.

CIRCUIT OPERATION
The service engine soon lamp is used to indicate to the operator a problem in the engine control system. If a problem is detected, the Powertrain Control Module (PCM) sends a message over the CCD Bus to the Body Control Module (BCM).

The BCM interprets this message and sends a signal to the instrument cluster to illuminate the lamp. This lamp also is illuminated, for a few seconds, when the ignition switch is moved from the OFF to the RUN position as a bulb check. The bulb check operation is controlled by the PCM.