Malfunction Indicator Lamp: Description and Operation

The Powertrain Control Module (PCM) controlled lamps are intended to alert the driver to an operating condition which may require immediate attention. ignition positive voltage powers the PCM controlled lamps. The PCM commands the lamps ON by applying ground to the lamp control circuit.

The Malfunction Indicator Lamp (MIL) is located on the instrument panel, CHECK ENGINE or SERVICE ENGINE SOON. The MIL performs the following functions:
^ It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as possible.
^ As a bulb check and system check, the MIL will illuminate with the ignition ON and the engine not running. When the engine is started, the MIL will turn OFF. If the MIL remains ON, the Self-Diagnostic System has detected a problem. If the problem goes away, the MIL will turn OFF in most cases, but a diagnostic trouble code will remain stored.
^ If the MIL is illuminated, then the engine stalls, the MIL will remain illuminated so long as the ignition is ON.
^ If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition is cycled OFF, then ON.

Perform the Powertrain On Board Diagnostic (OBD) System Check first, when the following conditions are present:
^ The MIL does not turn ON when the ignition is turned to the RUN position
^ The MIL remains ON while the engine is running
^ When you suspect a driveability problem
^ When you suspect an emissions problem