Check engine light is the most stressful dashboard light because it can mean anything from a $20 gas cap to a $3,000 engine job. Here are the eight causes we see most often at the shop, in rough order of frequency. If your light is flashing (not steady), pull over. A flashing CEL means an active misfire and continuing to drive can damage the catalytic converter.
1. Loose or cracked gas cap.
Causes an EVAP system leak code. The cheapest possible cause. Tighten the cap until it clicks several times, drive a few days, and the light often clears itself. Replacement gas cap if it is cracked: about $20.
2. Failing oxygen sensor.
Common on cars over 80,000 miles. The O2 sensor measures exhaust gas to help the engine adjust fuel mixture. When it fails, fuel economy drops and the CEL comes on. Replacement: $150 to $400 depending on which sensor and which car.
3. Catalytic converter inefficiency.
The cat is not converting exhaust gases as well as it should. Often caused by the O2 sensor failure above (which goes unfixed). Replacement: $800 to $2,500. Sometimes the cause is a problem upstream that is killing the cat; we always diagnose for that before recommending a replacement.
4. Mass airflow (MAF) sensor.
Measures the air entering the engine. When dirty, the engine misreads how much air is going in and runs rich or lean. Often cleanable for $50 to $100, replaced for $200 to $400 if cleaning does not fix it.
5. Spark plugs and ignition coils.
Worn plugs cause misfires. Worn coils too. Both are interval-based wear items. Plugs every 60,000 to 100,000 miles, coils as needed. Plugs: $80 to $250 for the set installed. Coils: about $80 to $200 each.
6. EVAP system leak (other than gas cap).
Cracked vapor hose, failed purge valve, failed vent valve. We smoke-test the EVAP system to find the actual leak. Repair: $150 to $400 typically.
7. EGR valve.
Carbon-clogged EGR valve causes rough idle and a CEL. Cleanable on some cars, replaced on others. $150 to $500.
8. Thermostat or coolant temp sensor.
Engine not reaching proper operating temperature trips the CEL. Thermostat replacement: $200 to $500. Coolant temp sensor: $100 to $200.
What to do.
Bring the car in. We pull the codes, run live data, and tell you the actual cause before quoting a repair. Replacing parts based on a code alone is the single most common way other shops waste customer money. Full diagnostic service details or call (415) 648-2226.