Why is my check engine light on?

The check engine light (malfunction indicator lamp, or MIL) is not a single problem. It is the engine computer telling you a monitor failed. The only way to know which monitor is to read the diagnostic trouble code.

Steady light vs flashing light

Steady usually means a stored emissions or drivability fault. You can often drive home, but you should scan the same day. Flashing almost always means a severe misfire. Unburned fuel is hitting the catalytic converter. Reduce load, avoid highways if the car is shaking, and diagnose immediately.

The 10-minute first response

1. Note whether the light is steady or flashing, and any symptoms (shake, smell, loss of power).
2. Check obvious items: loose gas cap, low oil, overheating gauge.
3. Scan for codes and freeze frame. Do not disconnect the battery to “reset” it — you will lose the snapshot that explains when the fault happened.
4. Search the code on this site (for example P0300 or P0420) so you understand the system before buying parts.

Why the light comes on after a fill-up

A loose or cheap gas cap can set EVAP codes such as P0442 or P0455. Tighten the cap until it clicks, then drive a few trips. If the light remains, scan — do not keep replacing caps forever.

Will the car fail inspection?

In most regions a commanded-on MIL is an automatic fail. Even after a repair, readiness monitors may need a drive cycle before they show complete.