Malfunction Indicator Lamp: Description and Operation

PURPOSE
The "service engine soon" Malfunction Indicator Lamp (MIL) is used by the PCM to signal the operator of the vehicle of a possible problem.
The MIL light is also used to relay diagnostic information during the diagnostic circuit check.


CIRCUIT OPERATION
When the ECM requires the indicator lamp to be turned "ON", it will supply a ground signal for the lamp. If the microprocessor is disabled for any reason, a ground for the check engine indicator lamp will be automatically provided.