Malfunction Indicator Lamp: Description and Operation

Malfunction Indicator Lamp (MIL)
OBD II regulations require that the vehicle's on-board computer lights up the Malfunction Indicator Light (MIL) on the instrument panel when the computer detects a malfunction in the emission control system/components or in the powertrain control components which affect vehicle emissions, or a malfunction in the computer. In addition to the MIL lighting up when a malfunction is detected, the applicable Diagnostic Trouble Code (DTC) prescribed by SAE J2012 is recorded in the ECM memory.

If the malfunction does not reoccur in 3 consecutive trips, the MIL goes off automatically but the DTCs remain in the ECM memory.