Check Engine Light

The check engine light, officially the malfunction indicator lamp or MIL, is the warning light a car's computerized engine-management system uses to flag a problem. That problem can be minor, like a loose gas cap, or serious, like worn spark plugs or engine damage. It usually reads "check engine," "service engine soon," or shows an engine symbol, and it lights up amber or red. When it comes on, the engine computer stores a fault code that can be read, usually with a scan tool. In the US, the EPA sets specific requirements for how it works.

The light generally has two modes. Steady means a fault that should be fixed soon to prevent further damage. Flashing means a severe problem that can make the car unsafe to drive, and it should get attention right away.

Some older cars had a single "engine" or "trouble" light that warned of imminent breakdown from low oil pressure, overheating, or a charging problem, and that kind of light was phased out in the mid-1980s to avoid confusion. The modern check engine light arrived in the early 1980s with computerized engine controls. Before standardized diagnostics, many cars could flash their fault codes through the light itself: jump two pins on GM's diagnostic connector, and the light would blink out code 12 as one blink, a pause, and two blinks. California required basic on-board diagnostics on new cars from 1988, and OBD-II, with its standardized connector and codes, became mandatory on all US passenger cars and gasoline light trucks under 8,500 pounds starting in 1996.