DiagnosticsPlan soon
Check Engine Light Diagnosis
A code tells you where the computer noticed a problem. It does not tell you what to replace.
Why it matters
The check engine light comes on when the engine computer sees a reading outside its expected range. A code like P0301 means "misfire on cylinder 1" — it does not mean "replace the coil." That misfire could come from a spark plug, a coil, an injector, a vacuum leak, or low compression, and the difference between those answers is often hundreds of dollars. Proper diagnosis is testing, not guessing.
Signs it is time
- Light is on steady
- Light is blinking or flashing — stop driving and get it checked
- Rough idle, misfire, or a noticeable loss of power
- Fuel economy dropped suddenly
- Failed an emissions test
If you wait
A blinking light usually means raw fuel is being dumped into the catalytic converter, which can destroy it — a repair that costs far more than the diagnosis. Ignored misfires also wash oil off cylinder walls and burn valves.