Malfunction Indicator Lamp (MIL)

The Malfunction Indicator Lamp (MIL) is required by On-Board Diagnostics (OBD II) that it illuminates under a strict set of guide lines.

Basically, the MIL is turned on when the powertrain control module (PCM)/engine control module (ECM) detects a DTC that will impact the vehicle emissions. The MIL is under the control of the Diagnostic Executive. The MIL will be turned on if an emissions-related diagnostic test indicates a malfunction has occurred. It will stay on until the system or component passes the same test, for three consecutive trips, with no emissions related faults.