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. A flashing light indicates a severe fault. This could potentially damage the catalytic converter if left uncorrected for an extended period. When the Check engine light is lit, the engine control unit stores a fault code related to the malfunction. The code can be retrieved with a scan tool and used for further diagnosis.
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.
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