warning light guide
Glow Plug Warning Light
The coiled spring symbol means your diesel pre-heaters or engine management system have detected an electrical or operating fault. You can usually drive home or to a garage, but the car may enter limp mode with reduced power.
What it looks like: The light appears as an amber coiled wire on your instrument cluster. It illuminates briefly when turning on the ignition, but staying lit or flashing while driving signals an active fault.
Updated 9 November 2026
Can I keep driving?
Service soon
Continuing to drive with an engine fault can damage the diesel particulate filter or leave you stranded in reduced power mode.
What to do right now
- 1Notice whether the light is steady or flashing and if the car has suddenly lost acceleration.
- 2If the car enters limp mode on a motorway, move safely into the left lane and take the next exit.
- 3Park in a safe spot, switch off the engine, wait two minutes, and restart to see if the light clears or remains.
Likely causes
Most likely first.
- Likely
Worn or failed glow plug
Over tens of thousands of kilometres, the internal heating element burns out or coats in carbon, stopping it from reaching the correct temperature to assist diesel combustion.
- Possible
Glow plug control module or relay failure
The electronic module controls the timing and power delivered to each plug, and cracked solder joints or failed internal switches prevent power from reaching the heater circuit.
- Possible
Engine management sensor fault
Many modern diesel engines flash this light to flag faults outside the glow circuit, including exhaust gas recirculation issues, turbo boost pressure deviations, or a failed brake pedal switch.
- Rare
Damaged glow plug wiring harness
Vibration, engine heat, and spilled diesel can make the plastic harness brittle or corrode the push-on connectors, breaking the electrical circuit.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Brake light check
Reverse up to a reflective wall or ask a helper to watch the rear of the car while you press the brake pedal. A faulty brake pedal switch commonly causes the glow plug light to flash on many diesel cars.
- 2
Cold start assessment
Pay attention to how the engine behaves when starting from cold in the morning. Excessive cranking, uneven idling for the first thirty seconds, or white exhaust smoke points towards failed glow plugs.
- 3
Visual fuse inspection
With the engine off and key removed, open the main fuse box under the bonnet. Consult the handbook to locate the glow plug fuse and inspect it for a broken metal filament or scorching.
- 4
Wiring loom inspection
Remove the plastic engine beauty cover if it lifts off freely and look along the cylinder head. Check that the wiring rail clipped onto each glow plug is firmly seated and free from broken wires or oil contamination.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. The total bill rises significantly if an old glow plug has seized inside the aluminium cylinder head and requires specialist engineering tools to extract.
Replace a set of glow plugs
- Do it yourself
- €40–120£35–105 in the UK
- At a garage
- €140–320£120–280 in the UK
Replace glow plug control module
- Do it yourself
- €60–190£50–165 in the UK
- At a garage
- €130–290£110–250 in the UK
Replace brake pedal switch
- Do it yourself
- €15–45£12–40 in the UK
- At a garage
- €70–150£60–130 in the UK
When to go to a garage
- The car enters limp mode and will not maintain ordinary traffic speeds safely.
- The warning light begins flashing while driving and is accompanied by heavy black or blue exhaust smoke.
- The engine turns over normally on the starter motor but refuses to catch and start.
- The glow plug warning light is accompanied by an oil pressure or coolant temperature warning light.
Questions drivers ask
Can I keep driving with the glow plug light on?
You can usually drive to your destination if the light is steady and the engine feels normal. If the light flashes or the car limits its speed, you should drive straight to a garage because the engine computer has detected an active powertrain fault.
Why does the glow plug light flash instead of staying solid?
A steady light usually indicates an issue within the pre-heating system itself. A flashing glow plug light is used by several manufacturers as an engine management alert to indicate an emissions, throttle, boost control, or brake switch error.
Will a glowing or flashing glow plug light cause an NCT or MOT fail?
The glow plug light itself is not a test item. If it also brings on the engine management light, that is an automatic fail on the UK MOT; on the Irish NCT, engine fault codes on their own give an Advisory Pass. A glow plug fault can also cause heavy exhaust smoke, and too much smoke is a fail on both tests.
Does clearing the fault code with an OBD scanner fix the problem?
Clearing the code only turns off the warning symbol temporarily until the car retests the circuit. The engine computer checks glow plug resistance on every cold start and monitors engine sensors constantly, so the light will return until the faulty part is replaced.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.