Check Engine Light On: What It Means and What to Do

The check engine light (also called the malfunction indicator lamp) comes on when the car's computer detects a problem in the engine, emissions or transmission systems and stores a fault code. The light itself does not say what is wrong. It only says that something is outside the normal range.

Steady light or flashing light?

• Steady light: the problem is not an emergency. You can usually drive to a garage, but do not ignore it for weeks.
• Flashing light: a serious misfire is happening and unburned fuel may be reaching the catalytic converter. Reduce speed, avoid heavy acceleration and stop as soon as it is safe. Driving on can damage the converter, which is one of the most expensive parts in the exhaust system.

The most common causes

• Loose or damaged gas cap. It breaks the seal of the fuel system. Tighten it until it clicks; the light may go out after a few drive cycles.
• Oxygen (O2) sensor. Worn sensors report the wrong air-fuel mixture, which raises fuel consumption.
• Spark plugs and ignition coils. Worn parts cause misfires, rough idle and hesitation.
• Mass airflow sensor. A dirty or failing sensor leads to poor acceleration and stalling.
• Catalytic converter. Often damaged as a result of another unresolved problem, such as misfires or a bad sensor.
• EVAP system leak. A small leak in the fuel vapor lines or a faulty purge valve.

What to do step by step

1. Check the dashboard for other warnings (oil pressure, temperature, battery) and note how the car feels: shaking, loss of power, strange smells or noises.
2. Check that the gas cap is tight.
3. If the light is flashing or the car runs rough, stop driving and arrange a tow or a short, gentle trip to a garage.
4. Have the codes read with a diagnostic scanner. A code points to a system or a circuit, not always to the failed part, so a good mechanic confirms the cause before replacing anything.

Why not just clear the code?

Clearing the code only turns the light off. If the fault is still there the light returns, and in the meantime the problem can get worse. A vehicle that has just been cleared may also not be ready for an emissions inspection, because the monitors need time to run again.

When should you see a mechanic?

Immediately if the light flashes, if the car loses power or overheats. Within a few days if the light is steady and the car drives normally. Browse the car electrics and diagnostics shops listed on our site, compare reviews from other drivers and call the garage before you go.