Check engine light

A check engine light or malfunction indicator lamp (MIL), is a tell-tale that a computerized engine-management system uses to indicate a malfunction or problem with the vehicle ranging from minor (such as a loose gas cap) to serious (worn spark plugs, engine problems or a faulty oil valve, etc.).

When the MIL is lit, the engine control unit stores a fault code related to the malfunction, which can be retrieved—although in many models this requires the use of a scan tool.

[6][7][8] Some vehicles manufactured from the late 1990s to mid 2000s have a MIL that illuminates based on the odometer reading, regardless of engine operation.

For example, in several Mazda models, the light will come on at 80,000 miles (130,000 km) and remain lit without generating a computer trouble code.

The MIL is also illuminated prior to starting the engine, along with other tell-tales on the dashboard, to demonstrate that the lamp is working and not burned out.

These burn differently from gasoline, and the EFI system may mistakenly interpret the oxygen sensor's readings as being incorrect.