Check engine light: what it means and what to do

The check engine light is the amber engine-shaped symbol on your dashboard. UK drivers usually call it the engine management light. It should come on for a few seconds when you start the car and then go out. If it stays on, the engine computer has found a fault and stored a code that explains what it is.

A steady light means "get this checked soon". A flashing light is different and more urgent: it usually means the engine is misfiring badly. If yours is flashing, read our guide to the flashing check engine light instead.

Likely causes, most common first

1. Oxygen sensor

   The oxygen sensor measures how much unburned oxygen is in the exhaust, so the engine can get the fuel mix right. It is one of the most common check engine repairs in CarMD's yearly index. A failing one hurts fuel economy and, left alone, can damage the catalytic converter.

2. Loose or faulty fuel cap

   The cheapest fix of all. A cap that is loose, cracked or missing its seal lets fuel vapor escape and triggers an emissions fault. Tighten it until it clicks and give it a few drives to see if the light clears.

3. Spark plugs or ignition coils

   Worn plugs and failing coils cause misfires. You may feel rough running, hesitation or poorer fuel economy. A bad misfire makes the light flash.

   Read the fix guide →

4. Mass airflow sensor

   This sensor measures the air going into the engine. When it fails, the car can idle roughly and accelerate poorly. Sometimes cleaning it is enough.

5. Catalytic converter

   The most expensive common cause. A converter usually fails because something else went wrong first, such as a long-running misfire or a bad oxygen sensor, so a good garage fixes that cause too.

6. Other sensors and emissions parts

   EGR valves, diesel particulate filters (DPF), fuel injectors and many other sensors can all set the light. A weak battery can also cause electrical faults that show up here.

The only way to know which one it is: read the fault code. You can do this yourself with an OBD2 scanner, which plugs into a port under the dashboard on the driver's side. AAA says these work on most cars made after 1996. Check your owner's manual if your car is older or was built for another market. See our guide to the best OBD2 scanners. A code points to a system, not always a single part, so do not replace parts on the code alone.

Can I keep driving?

• Steady light, car feels normal: usually yes, for short distances. Book a check soon and avoid hard acceleration and high revs. The longer you leave it, the more likely a small fault damages something bigger.
• Steady light plus symptoms (shaking, loss of power, smoke, a strong smell): pull over safely, switch off the engine and call for help.
• Flashing light: slow down and stop driving as soon as it is safe. See flashing check engine light.
• Red engine light (some cars use one): treat it as stop now.

Check what else is on the dashboard too. If the red oil pressure or temperature light is on, those come first: see the oil pressure light and engine temperature light guides.

What it costs to fix

These are the five most common check engine repairs in CarMD's 2022 Vehicle Health Index, which covers US repairs carried out in 2021. They are national averages for parts and labor; prices have risen since, and your car may differ.

DIY or garage?

How to prevent it

• Close the fuel cap until it clicks every time you fill up.
• Replace spark plugs at the interval in your owner's manual.
• Do not ignore a light that comes on and goes off again: the code is still stored and can be read.
• Fix misfires quickly, before they damage the catalytic converter.
• Keep up with regular servicing; use our service tracker if it helps.