Steady or flashing?
- Steady light: the car has stored a fault. If it drives normally and no other warning lights are on, you can usually keep driving for a short time, but read the code soon.
- Flashing light: the engine is misfiring badly enough to overheat the catalytic converter. Ease off the gas, avoid hard acceleration and pull over when it's safe, then don't keep driving: have the car checked or towed. Driving on can ruin the converter.
Stop driving if you notice any of these
- The temperature gauge is in the red or a coolant warning comes on.
- The red oil pressure light comes on, or the engine starts knocking.
- The red brake warning light is on, or the brake pedal feels soft or different.
- The battery light is on and the lights or gauges start to dim.
- Steering suddenly gets heavy, or you see smoke or smell burning.
- The car is stuck in one gear or can't keep up with traffic.
In any of these cases, pull over somewhere safe, switch the engine off and have the car checked before driving further.
Check these first
- The gas cap. If the light came on after you filled up, make sure the cap is there, its seal isn't cracked and it clicks when tightened. The light can take several days of driving to go off.
- How the car behaves. Note any rough idle, shaking, loss of power, odd shifting or new noises, and when the light first came on. It helps whoever diagnoses the car.
Read the code
Gasoline cars and light trucks sold in the US since the 1996 model year have an OBD-II port under the dashboard, usually near the steering column; diesels followed shortly after. Plug in an inexpensive code reader or Bluetooth adapter, or ask an auto parts store: many will read codes for free. Basic readers show engine codes; ABS, airbag and many transmission codes need a scanner that can read those modules.
Then look up the code here:
Common reasons the light comes on
- P0456 Small EVAP leak, often the gas cap
- P0455 Large EVAP leak or a missing gas cap
- P0300 Engine misfire
- P0420 Catalytic converter efficiency
- P0171 Engine running lean
- P0101 Airflow sensor reading
- P0128 Thermostat stuck open
- P0011 Camshaft timing, often low or dirty oil
- P0700 Transmission fault stored in the transmission computer
Or browse all codes by system.
Should I just clear the code?
Clearing the code turns the light off but doesn't fix anything: if the fault is still there, the code comes back. Clearing also resets the car's readiness monitors, and many inspection stations turn a car away until those self-tests have run again over several days of driving. Fix the cause first, then clear.
Will I pass an emissions inspection?
In states that test cars by scanning the OBD system, a check engine light that's on usually means an automatic fail, and so do too many readiness monitors that aren't ready yet. Rules differ by state and county, so check your local program before you go.