Malfunction Indicator Light MIL

When a malfunction occurs, the MIL remains illuminated as long as the fault is detected and goes off once normal conditions return, leaving a Diagnostic Trouble Code (DTC) stored in the ECM memory. Circuits are monitored for continuity, shorts, and in some cases, normal parameter range.

The Malfunction Indicator Light (MIL) is also a visual inspection item in most emissions inspection and maintenance programs U/M), allowing the emissions inspector to make a quick visual determination whether the engine control/emissions system is functioning normally. During the visual inspection phase of the I/M test, the inspector must observe the MIL during a "key on bulb check" and again with the engine running. The MIL should be on during the bulb check and go off when the engine starts. When a vehicle passes this check, it is highly probable that the engine control system is functioning normally.

Although the OBD regulation applies only to California emissions certified vehicles, some or all of the OBD system features are found on Federal emissions certified vehicles as well.

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