Why is my check engine light on?
A steady check engine light means the car's computer has detected a fault in the engine, emissions or fuel system and stored a trouble code. Check that the gas cap is tight first. If the car drives normally, it's usually safe to drive for a few days, but get the code read and the cause tested soon. If the light is flashing, slow down and get it checked right away.
What the light means
Since 1996, every car sold in the US has an onboard diagnostic system (OBD-II) that watches the engine and emissions systems. When a reading stays outside its normal range, the computer stores a trouble code and turns on the check engine light, also called the malfunction indicator lamp.
The light covers hundreds of possible faults. A few are trivial. A few, if ignored, can turn a small repair into a much bigger one.
Steady or flashing?
| Light | What it means | What to do |
|---|---|---|
| Steady | A fault was detected and stored | Drive gently and get it checked within days |
| Flashing | An active misfire that can damage the catalytic converter | Slow down, avoid hard acceleration, and get it checked right away |
| Steady, and the car runs badly | A fault that's affecting how the engine runs | Get it checked as soon as possible |
Common causes
- Loose or worn gas cap (an EVAP system leak)
- Oxygen or air-fuel ratio sensor failure
- Misfires from worn spark plugs or ignition coils
- Vacuum leaks from cracked hoses or intake gaskets
- Mass airflow sensor that's dirty or failing
- Catalytic converter efficiency codes (often P0420)
- Thermostat stuck open (common in colder months)
- Variable valve timing faults, especially on some Honda engines
What to do now
- Check the gas cap. Remove it and tighten it until it clicks.
- Notice how the car drives. Rough idle, hesitation, poor fuel economy or new noises all help with diagnosis.
- Look for other warning lights. A check engine light with a temperature or oil light is more serious.
- Get the code read and the cause tested. The code points to a system; testing finds the part. That's the difference between fixing it once and replacing parts on a guess.
Pull over and call us if the check engine light comes on with the temperature gauge in the red, the oil pressure light, or a loss of power.
From drivers who came to us
Dylan quickly fixed an oxygen sensor problem… I trust him to fix only what needs fixing.
I had my 2009 prius "check engine" light on that somehow caused smog test failing… The check engine light issues were fixed. Smog test was passed.
They were very friendly and offered to check my vehicle out and check the engine with the codes… My car worked perfect after I left.
Excerpts from verified customer reviews on Yelp.
More questions
Can a loose gas cap really turn on the check engine light?
Yes. The evaporative emissions system checks itself for leaks, and a loose or worn cap is a common cause. Tighten it until it clicks; the light may take several drives to go off on its own.
Will the check engine light reset itself?
Sometimes. If the fault goes away, the light usually turns off after a few drive cycles. The code stays stored for a while, which is why a scan can still show it.
Can I pass a California smog check with the check engine light on?
No. A lit check engine light is an automatic failure in California. More on smog failures.
Should I just clear the code?
Clearing it doesn't fix anything, and it erases information that helps find the cause. It also resets the car's emissions self-tests, which blocks a smog check until they complete again.