Published November 2026.
The glow plug light is an amber symbol that looks like a coiled spring or a curly wire. You’ll only see it on a diesel. Most of the time it’s completely normal: it comes on when you turn the key and goes out when the engine is ready to start. If it stays on, comes back while you’re driving or flashes, something needs looking at. Here’s what each means.
Glow plug light: what to do now
- On for a few seconds when you switch on the ignition: normal. Wait for it to go out, then start the engine. It may take a little longer on a cold morning.
- Stays on after the engine has started, or comes on while driving: the car has found a fault, either with the glow plugs or, on some cars, the engine. Get it checked soon.
- Flashing: treat it as an engine fault warning. If the car is losing power or running badly, pull over when it’s safe and call your breakdown provider; otherwise get the fault code read as soon as you can.
What do glow plugs do?
Petrol engines use spark plugs to ignite the fuel. Diesels don’t: they rely on the heat created by squeezing air very hard. When the engine is cold, that heat isn’t enough on its own, so each cylinder has a glow plug, a small electric heater that warms the combustion chamber before and just after you start. The light tells you they’re heating up.
As What Car? explains, the light typically stays on for a few seconds after you turn the ignition on. Modern diesels heat up very quickly, so you may barely see it in mild weather.
Why is my glow plug light staying on?
If the light stays on after the engine has started, or comes on later, What Car? says there might be an issue with the plugs. Common causes are:
- One or more worn-out glow plugs. They wear gradually, and one failing plug is enough to set the light off.
- A faulty glow plug relay or control unit, which switches power to the plugs.
- Wiring faults to the plugs or the relay.
- An engine fault, on cars where the glow plug light doubles as the diesel engine warning light.
Why is my glow plug light flashing?
On some diesels, including many Volkswagen Group cars, the same coil symbol doubles as the diesel engine management light; Volkswagen UK calls it the glow plug/diesel engine management light. On those cars, a flashing or lit light while driving means the engine computer has stored a fault, which may have nothing to do with the glow plugs. Common culprits include fuel pressure, turbo boost and emissions faults. The engine may also go into a reduced-power limp mode to protect itself.
The only way to know is to have the fault code read. Our guide to the engine management light explains how fault codes work.
Signs your glow plugs are wearing out
- The engine is slow to start, or needs several attempts, especially on cold mornings.
- It runs roughly, knocks or smokes white or grey for the first minute after a cold start.
- The glow plug light stays on longer than usual, or comes back on after starting.
Glow plugs are usually replaced as a set. On some engines they can seize in the cylinder head, which makes the job much longer and dearer, so it’s worth using a garage that knows your engine. Ask for a quote once the fault has been diagnosed.
Can I drive with the glow plug light on?
If the engine starts and runs normally, a worn glow plug won’t stop you driving, but the car will get harder to start as the weather gets colder, and you risk being stranded. If the light is acting as an engine warning, or the car is down on power, don’t keep driving it: an engine fault can get worse and more expensive. When in doubt, get it checked.
Is a glow plug light an MOT fail?
Glow plugs aren’t on the MOT checklist as such. But on cars where the glow plug light doubles as the engine warning light, the MOT inspection manual treats an engine malfunction indicator (MIL) that is inoperative or indicating a malfunction as a major defect on a diesel (section 8.2.2.2), so the car would fail.
Buying a used diesel?
- Ask to start the car from cold. Watch the glow plug light come on and go out, and check the engine starts cleanly without lots of smoke.
- Check that no warning lights stay on once the engine is running.
- Check the MOT history for emissions failures and advisories with our free MOT history check.
Our guide to dashboard warning lights covers the other lights to look for.
Frequently asked questions
Is it normal for the glow plug light to come on?
Yes, for a few seconds when you switch on the ignition of a diesel. Wait for it to go out, then start the engine.
What does a flashing glow plug light mean?
On many diesels it doubles as the engine warning light, so flashing means the engine computer has found a fault. Get the fault code read.
Can a bad glow plug stop a car starting?
It can make a diesel slow or hard to start, especially in cold weather. A warm engine usually still starts.
Do petrol cars have a glow plug light?
No. Only diesels have glow plugs.
Will a glow plug light fail an MOT?
Not as a glow plug fault. But if it’s acting as the engine warning light and shows a fault, the car fails.
- What Car?: car dashboard warning lights explained (3 March 2025)
- Volkswagen UK: glow plug/diesel engine management light
- GOV.UK: MOT inspection manual, section 8.2 emissions
- Engine management light: what it means and how to fix it
Check a car’s MOT history, including any failures for warning lights, with our free MOT history check.
