Check Engine Light Is On

What's happening

Most likely causes

1. 1

   Worn spark plugs or ignition coils

   The number one cause. Misfires under load that clear at idle are classic coil failure.

2. 2

   Vacuum leak

   Unmetered air leans the mixture; listen for hissing around the intake.

3. 3

   Dirty mass airflow sensor

   Skews fueling and triggers lean codes alongside the light.

4. 4

   Failing catalytic converter

   A clogged converter chokes the engine and sets efficiency codes.

5. 5

   Loose gas cap

   The classic free fix. An EVAP leak from a loose cap sets the light with no symptoms at all.

How to diagnose it — in order

1. 1

   Scan the codes first. The code tells you which system to chase; guessing without it wastes money.

2. 2

   If the light is steady and the car drives normally, check the gas cap and schedule diagnosis soon.

3. 3

   Note when it came on: after fueling points at EVAP, under acceleration points at misfire or fueling.

4. 4

   Do not clear the code before diagnosis; the freeze-frame data shows exactly when it set.

Trouble codes tied to this symptom

If your scanner shows one of these, read its dedicated trouble-code page for the full diagnosis.