Malfunction Indicator Lamp

A Malfunction Indicator Lamp (MIL) is an indicator of the internal status of a car engine. It is found on the instrument console of most automobiles. When illuminated, it is typically either a red or amber 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, such as catalytic converter problems or engine misfire). 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 is usually labeled with the text Check Engine, Service Engine Soon, Check Engine Soon, or a picture of an engine.

The MIL became required on passenger cars in the United States due to emission control legislation in California, with the intention that the light would illuminate if there was a problem which would cause the vehicle to have excessive pollutant emissions. The owner would be aware that the emission control system needed to be serviced, and would be prevented from renewing their registration in the state of California.[citation needed] In most states and regions that require emissions inspections, a lit MIL on an OBD-I or OBD-II vehicle will cause the vehicle to fail the inspection


"Trouble" indicator
Some older vehicles had a single indicator labeled "Trouble" or "Engine"; this was not an MIL, but a warning 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.

Odometer triggering
Some vehicles made in the late 80s and early-to-mid 90s have a MIL that illuminates based on the odometer reading, regardless of what is going on in the engine. For example, in several Mazda models, the light will come on at 80,000 miles and remain lit without generating a computer trouble code. This was done in order to remind the driver to change the oxygen sensor.

All American production 1973-1976 Chrysler/Plymouth/Dodge/Imperial cars had a similar odometer-triggered reminder: "Check EGR", which was reset after service at a Chrysler dealership