Why Your Check Engine Light Is On: 7 Most Common Causes

That glowing check engine light can mean anything from a loose gas cap to a failing catalytic converter. Here are the seven most common triggers and what they'll cost to fix.

1. Loose or Damaged Gas Cap ($0-$25)

The most common and cheapest fix. Tighten it until it clicks. If the light persists, replace the cap for under $25.

2. Oxygen Sensor Failure ($150-$400)

A failing O2 sensor reduces fuel efficiency by up to 40%. Most cars have 2-4 sensors.

3. Catalytic Converter ($1,000-$2,500)

The most expensive common repair. Often fails due to neglected spark plugs or ignition coils.

4. Mass Airflow Sensor ($200-$400)

Measures incoming air to determine fuel mixture. Often fixable by cleaning with MAF cleaner spray ($8).

5. Spark Plugs/Ignition Coils ($100-$350)

If your engine misfires or hesitates, these are likely culprits.

6. EVAP System Leak ($100-$600)

Related to fuel vapor recovery. Often a cracked hose or faulty purge valve.

7. Thermostat ($150-$300)

If your engine runs hot or cold, the thermostat may be stuck.

Don't ignore the light — get the code read free at any auto parts store.