By what you're feeling
Check engine light + a symptom
The light is on, and so is something else — a smell, a shake, a warning. Pick the symptom closest to yours and read the matching guide.
How the light behaves
After a service event
With a noticeable symptom
- Plus rough idleMisfire, vacuum leak, dirty throttle body, or IAC valve.
- Plus car shakingAlmost always an engine misfire.
- Plus stallingDiagnose by when it stalls: idle, deceleration, or under load.
- Plus loss of powerCatalyst clogged, turbo, throttle body, or multiple misfires.
- Plus reduced power / limp modeECM-commanded protection. Drive at low speed to a safe stop.
With a smell
With another warning light
Don't see your symptom?
Two other pillars cover the rest: the safe-to-drive decision tree if the question is severity, and the by-vehicle hub if the question is brand-specific.