Malfunction Indicator Light

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Malfunction Indicator Light (MIL) alerts the driver that PCM has detected an OBD-II emission related component or system fault. When this occurs, an OBD-II DTC will be set. MIL is located on instrument cluster and is labeled CHECK ENGINE, SERVICE ENGINE SOON or uses an ISO standard engine symbol. See Fig 1. Power is supplied to MIL whenever ignition switch is in RUN or START position. MIL will remain ON in RUN or START mode as a bulb check during instrument cluster proveout for about 4 seconds. MIL will remain on after bulb check for the following reasons:

PCM illuminates MIL for an emission related concern and a DTC will be present.
Instrument cluster will illuminate MIL if PCM does not send a control message to instrument cluster.
PCM is operating in Hardware Limited Operation Strategy (HLOS). See HARDWARE LIMITED OPERATION STRATEGY under POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS.
MIL circuit is shorted to ground.

If MIL remains off (during bulb check), possible causes are; bulb is damaged or MIL circuit is open. To turn off the MIL after a repair, a reset command from scan tool must be sent, or 3 consecutive drive cycles must be completed without a fault. If MIL blinks at a steady rate, a severe misfire condition could possibly exist. If MIL blinks erratically, an intermittent open in B+ circuit to bulb or an intermittent short to ground in MIL circuit may exist. Also, PCM can reset while cranking if battery voltage is low.

Fig 1: ISO Standard Engine Symbol

Courtesy of FORD MOTOR CO.