Indicator lamp

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 fault, that 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 diagnosis may be retrieved without a scan tool in some car models. The malfunction indicator lamp usually bears the legend check engine, service engine soon, 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 would flash the codes, for instance (blink) (pause) (blink) (blink) for code 12. Some manufacturers, such as Honda, retained this feature even after OBDII.

Some older vehicles had a single indicator labeled "trouble" or "engine"; this was not a MIL, but an "idiot light" meant to indicate serious trouble with the engine (low oil pressure, overheating, or charging system problems) and an imminent breakdown. This usage of the "engine" light was discontinued in the mid-1980s, to prevent confusion with the MIL. The MIL is commonly referred to today as the "check engine light" or the "service engine soon light".

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Wikipedia