Glow Plug Light Flashing: What It Means on a Diesel
A flashing glow plug (coil) light on a diesel rarely means glow plugs. On most makes it is the engine warning you of a fault serious enough to trigger limp mode, with reduced power to protect itself. You can usually drive gently to a garage, but not on a motorway. Diagnosis is £50-95, and common fixes run from £80 to around £560.
What it means
The coil symbol normally glows steadily for a few seconds before you start a cold diesel, that is the glow plugs preheating and it is completely normal. A flashing coil light while driving is a different message altogether. On most diesels, VW group, Ford, Vauxhall, Peugeot, Citroen and others, a flashing glow plug light means the engine management has detected a fault and has usually put the car into limp home mode, capping power and revs to protect the engine.
Confusingly, the actual fault is often nothing to do with glow plugs. The flashing lamp is just how many diesels signal an engine fault, sometimes alongside the engine management light, sometimes on its own.
Can you keep driving?
Amber, with limits. In limp mode the car is protecting itself, so short, gentle driving to a garage is generally fine. What is not fine is a motorway run, the car may refuse to pull above 50 or so and can feel dangerously flat when you need acceleration to join or overtake. Sometimes switching off, waiting a minute and restarting clears limp mode temporarily, that is useful for getting off a junction, but the fault is still logged and it will come back. Get the codes read within days.
Most common causes
- Turbo boost faults, a stuck actuator, split intercooler hose or seized vanes
- EGR valve stuck or clogged with carbon
- Faulty sensor, MAF, MAP or camshaft/crankshaft sensors
- Injector faults or fuel pressure problems
- DPF blockage on higher-mileage or short-trip diesels
- Actual glow plug or glow plug module failure, usually a steady light or hard cold starting rather than flashing
What it costs to fix
| Repair | Typical UK independent garage price |
|---|---|
| Diagnostic check | £50-95 |
| Glow plugs (set) | £80-200 |
| Glow plug control module | £100-250 |
| Boost/intercooler hose | £60-160 |
| EGR valve | £280-560 |
| DPF clean | £250-500 |
Because the causes range so widely, this is a light where paying for a proper diagnostic saves money almost every time.
Will it fail the MOT?
The glow plug lamp itself is not a checked item in the MOT. But the fault behind it usually stores engine management codes, and if the engine management light is on during the test, that is a major defect and a fail. A diesel in limp mode may also struggle to complete the metered smoke test, and excessive smoke is a fail. So while the flashing coil light is not technically an MOT item, the car behind it often fails anyway.
Common questions
The light stopped flashing after I restarted the car. Is it fixed?
No, restarting just resets limp mode. The fault code that triggered it is still stored, and the condition that caused it, a boost leak, EGR or sensor fault, is still there. It will flash again, usually at the least convenient moment. Have the codes read while the garage can still see the history.
Do I need new glow plugs then?
Probably not for a flashing light. Failed glow plugs typically cause hard starting and rough running on cold mornings, sometimes with a steady lamp or a stored code, and a set costs £80-200 to replace. The flashing light is the engine's general fault signal, and the diagnostic will tell you what actually tripped it.
Why does it only flash when I accelerate hard or go uphill?
That pattern points strongly at a boost fault. Under hard acceleration the turbo works hardest, and if a hose is split or the actuator is sticking, the ECU sees boost pressure out of range and trips limp mode. It is one of the most common causes and often one of the cheaper fixes if it is just a hose.