Malfunction Indicator Lamp: Description and Operation

PURPOSE
The Malfunction Indicator Lamp (MIL) alerts the driver that the Powertrain Control Module (PCM) has detected an OBD II emission related component or system malfunction. When this occurs, an OBD II Diagnostic Trouble Code (DTC) will be set.






OPERATION
The MIL is labeled CHECK ENGINE or SERVICE ENGINE SOON. Power is supplied to the MIL whenever the ignition switch is in the run or crank position. The MIL remains On in the Run/Crank mode as a bulb check until the Profile Ignition Pickup (PIP) signal is detected.

DIAGNOSTIC NOTES
The light may also be On due to a short to ground of the MIL circuit, or operation in the Hardware Limited Operation Strategy (HLOS).

In addition, the MIL will remain On if the MIL was On when the vehicle was last shut down. If the MIL does not turn Off while the engine is cranking, it could indicate the PCM is not receiving PIP signals or the MIL circuit is shorted to ground. If the MIL blinks, there is a severe misfire or an intermittent in the MIL circuit.

To extinguish the MIL after a repair, a reset command from the scan tool must be sent, or three consecutive drive cycles must be completed without a malfunction. Refer to Computers and Control Systems \ Testing and Inspection \ Procedures \ Trips and Drive Cycles for more information. If the MIL never comes On or the vehicle is a no-start, go to Powertrain Management \ Testing and Inspection \ Diagnosis by Symptom Index.