By Shore Brothers Automotive, Cape May Court House, NJ · Published
The short answer
A steady check engine light means your car's computer found a problem: it's usually safe to keep driving, but get it checked soon. A flashing light means a serious engine misfire that can damage the catalytic converter: slow down, limit driving and get it looked at right away.
Steady vs. flashing
- Steady light: something is outside normal range. It's rarely an emergency, but don't ignore it. Small problems can turn into bigger ones, and a lit light can cause a failed emissions inspection.
- Flashing light: the engine is misfiring badly enough to send unburned fuel into the exhaust, which can overheat and ruin the catalytic converter. Reduce speed, avoid heavy loads and get it checked as soon as possible.
- If the light comes on with overheating, low oil pressure, smoke or a big loss of power, pull over safely and call for help.
The most common causes
- A loose, cracked or missing gas cap.
- A failing oxygen (O2) sensor.
- Worn spark plugs or a bad ignition coil (often the cause of a flashing light).
- A small leak in the evaporative emissions (EVAP) system.
- A dirty or failing mass airflow sensor.
- A catalytic converter that's no longer working efficiently.
Reading the code isn't the same as a diagnosis
A code tells you which system reported a problem, not which part failed. For example, a code for a lean condition can come from a vacuum leak, a weak fuel pump or a dirty sensor. Replacing parts based on the code alone often wastes money.
A real diagnosis looks at live engine data and tests the related parts to find the actual cause, then gives you a clear estimate before any work is done.
Don't just clear the code
Clearing the light with a scanner or by disconnecting the battery doesn't fix anything, and it resets the car's self-tests. If you go for an emissions inspection before those tests finish running, the car can fail even if the light is off.