Malfunction Indicator Lamp: Description and Operation

The malfunction indicator lamp (MIL) or (SERVICE ENGINE SOON telltale) is in the instrument cluster and it informs the driver that a problem has occurred and that the vehicle should be taken in for service as soon as reasonably possible.

As a bulb and system check, the lamp will come On with the key On and the engine not running. When the engine is started, the telltale will turn Off. If the telltale remains On, the self-diagnostic system has detected a problem. If the problem goes away, the lamp will turn Off in most cases after three trips, but a diagnostic trouble code will remain stored in the ECM.

If the telltale remains On while the engine is running, or when a malfunction is suspected due to a driveability problem, an on-board diagnostic system check (OBD system check) must be performed. These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely. (Refer to the "Vehicle On Board Diagnostic [OBD] System Check" in Body Electrical Diagnostics. Initial Inspection and Diagnostic Overview)