A malfunction indicator lamp (MIL), also known as a check engine light, is a tell-tale to indicate malfunction of a computerized engine management system. It is found on the instrument panel of most automobiles. When illuminated, it is typically either an amber or red color. On vehicles equipped with OBD-II, the light has two stages: steady (indicating a minor fault such as a loose gas cap or failing oxygen sensor) and flashing (indicating a severe faultthat could potentially damage the catalytic converter if left uncorrected for an extended period).
When the MIL is lit, the engine control unit stores a fault code related to the malfunction, which can be retrieved with a scan tool and used for further diagnosis. The malfunction indicator lamp usually bears the legend or a pictogram of an engine. In the United States, specific functions are required of the MIL by EPA regulations. The MIL appeared in the early 80s along with computerized engine controls.
Even the earliest systems, such as GM’s CCC (Computer Command Control) system, had self-diagnosis functionality. When the computer detected a fault, it illuminated the MIL. Up until OBDII, on most cars, the MIL could output codes; when two pins on the ALDL are jumped, the light will flash the codes, for instance, (blink) (pause) (blink) (blink) for code 12. Some manufacturers, such as Honda, retained this feature even after OBDII.