Flashing check engine light: is it safe to drive?

A check engine light that blinks on and off while you drive is the engine computer's way of shouting. A steady light means "book a check". A flashing light means "something is being damaged right now". If your light is steady rather than flashing, see our main check engine light guide.

What to do right now

1. Ease off the accelerator. Do not brake hard or swerve; just reduce speed and load on the engine.
2. Find a safe place to stop and switch off the engine. On a motorway or highway, that means the next exit, services or hard shoulder, not a live lane.
3. Look at the rest of the dashboard. If the red oil pressure or temperature light is also on, the engine needs to be switched off straight away.
4. Call for help. Ring your breakdown service or a garage. Getting the car recovered protects the catalytic converter, the most expensive part at risk.
5. Do not keep restarting the engine to see if the light has gone. Leave it to a mechanic to read the fault code.

The AA says that with a flashing amber light, or warning signs such as noises, lost power, smoke or strange smells, you should pull over safely, switch off the engine and report a breakdown. AAA gives the same advice.

Likely causes, most common first

1. Worn or fouled spark plugs

   Petrol engines need a strong spark in every cylinder. A worn or fouled plug fails to fire, and that cylinder misfires. You often feel it as shaking or stuttering, worst when accelerating.

2. Failing ignition coil

   Many modern engines have a separate coil for each cylinder. When one fails, that cylinder stops firing properly. The AA names plug and coil faults as typical reasons for a flashing light on petrol cars.

3. Fuel injector problem

   A clogged or leaking injector gives one cylinder the wrong amount of fuel, which can also cause a misfire.

4. Catalytic converter already damaged

   The AA notes a flashing light can mean a critical problem affecting the catalytic converter. Often the converter is the victim of a misfire, not the cause.

5. Other engine faults

   Less often, a vacuum leak, low fuel pressure or a mechanical problem inside the engine causes the misfire. A mechanic will check these once the simple causes are ruled out.

The fault code tells the mechanic which cylinder is misfiring, if the computer can tell. An OBD2 scanner can read it at the roadside, but it does not make the car safe to drive.

What it costs to fix

Average US repair costs from CarMD's 2022 Vehicle Health Index (repairs done in 2021, parts and labor):

DIY or garage?

How to prevent it

• Change spark plugs at the interval in your owner's manual.
• Get a steady check engine light looked at before it turns into a flashing one.
• Do not ignore rough running, hesitation or a sudden drop in fuel economy.
• Use the fuel grade your owner's manual specifies.