Is it safe to drive with the check engine light on?

It depends on the fault. A flashing check engine light means stop driving immediately — you are damaging the engine. A solid light means you can usually drive to a garage but should not delay. Never ignore it on a car you are about to buy.

How much does it cost to fix a check engine light?

Anywhere from £50 (fix a loose fuel cap) to £2,000+ (replace a catalytic converter). You must diagnose the fault first to know the cost. Never buy a car with a check engine light without getting a quote for the repair.

Will a car fail its MOT if the check engine light is on?

If the OBD fault relates to emissions or safety systems, yes. If it is a non-emissions fault, the car may pass MOT but is still unreliable. Always clear the fault before MOT and get any underlying issue fixed.

Check Engine Light Explained

Q: What does the check engine light mean?

The check engine light indicates the onboard diagnostic system (OBD) has detected a fault in the engine, emissions system, or transmission. It does not mean the engine is about to fail — some faults are minor, others are serious.

What Is the Check Engine Light?

The check engine light (also called the engine management light) is part of the car's onboard diagnostic system (OBD). When the system detects a fault in the engine, emissions system, or transmission, it illuminates the warning light.

The light can mean anything from a loose fuel cap (£0 to fix) to a failing catalytic converter (£1,500–£2,000 to replace). The only way to know is to connect a diagnostic scanner and read the fault code.

Solid Light vs Flashing Light

Solid light: A steady check engine light means a fault has been detected, but it is not immediately critical. You can usually drive to a garage, but do not delay. The fault should be investigated within days.

Flashing light: A flashing check engine light is serious — it means the engine is misfiring or the emissions system is failing in a way that damages the catalytic converter. Stop driving immediately and have the car towed to a garage. Continuing to drive risks severe engine damage.

What Fault Codes Actually Mean

Every fault detected gets a code (e.g. P0171, P0300). These codes tell you which system is faulty. Common examples:

P0171 / P0174 (fuel system): Engine is running too lean. Possible causes: oxygen sensor failure, vacuum leak, fuel injector issue. Repair cost: £200–£800
P0300 (random misfires): Engine is misfiring. Possible causes: bad spark plugs, coil pack failure, fuel issue. Repair cost: £100–£600
P0420 (catalytic converter): Catalytic converter efficiency below threshold. Possible causes: converter failure, oxygen sensor, engine running too rich. Repair cost: £400–£2,000
P0141 (oxygen sensor): Oxygen sensor fault. Usually fixable with sensor replacement. Repair cost: £150–£400

If the Car Has a Check Engine Light

Do not buy without a diagnosis. Ask the seller to have the car scanned at a garage (costs £50–£100) and show you the fault codes. If they refuse, walk away — they are hiding the cost of repair.

Once you have the fault codes, get a quote for repair. For emissions-related faults (catalytic converter, oxygen sensors), budget £400–£2,000. For ignition/fuel-related faults, budget £100–£800.

Use the repair cost as negotiating leverage: "The diagnostic shows a (fault). The repair will cost £X. I am reducing my offer by £(X+10%) to cover it."

Can I Clear the Light Myself?

You can clear the fault code using an OBD scanner (~£50), and the light will go out. But clearing the code does not fix the underlying fault — it will usually come back within 50–100 miles of driving. Never buy a car where the light "mysteriously" came on and off — this is a classic tactic to hide problems.

MOT and Check Engine Light

If the fault is emissions-related (catalytic converter, emissions system), the car will likely fail MOT. The light being on is a failure by itself if it is an emissions fault. Non-emissions faults may allow a pass, but the car is still unreliable.

Before buying any used car, check the free MOT history. Repeated emissions failures or engine-related advisories in the MOT record are a warning sign that engine problems predate your ownership.