Malfunction Indicator Lamp (MIL)

The malfunction indicator lamp (MIL) "SERVICE ENGINE SOON" telltale lamp is in the instrument cluster and has the following functions:
^ It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.
^ It displays diagnostic trouble codes stored by the PCM which help the technician diagnose system problems.
^ It indicates Open Loop or Closed Loop engine operation.

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 lamp will turn Off. If the lamp remains On, the self-diagnostic system has detected a problem. If the problem goes away, the lamp will go out in most cases after 10 seconds, but a diagnostic trouble code will remain stored in the PCM.

When the lamp 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 On Board Diagnostic System Check [OBD System Check]) Testing and Inspection