Malfunction Indicator Lamp: Description and Operation

OBD II regulations require that the vehicle's on- board computer lights up the Malfunction Indicator Lamp (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 Codes (DTC) prescribed by SAE J2012 are recorded in the ECM memory.

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