Glow plug light flashing: what your diesel is trying to tell you
Updated · Autofy
Glow plug light flashing: what your diesel is trying to tell you
On a diesel, the glow plug light (an amber coil) coming on for a few seconds with the ignition is normal: the plugs heat the combustion chamber, and in cold weather you wait for it to go out before cranking. A light that flashes while driving, however, signals an engine management fault on many makes, often with a loss of power as the car goes into limp mode. You can usually drive gently to a garage, where a diagnostic scan reads the fault code.
Steady at startup: normal
When cold, a diesel needs help to fire. The glow plugs heat the combustion chamber for a few seconds, and the coil-shaped light stays on meanwhile.
The good habit, especially in winter: turn the ignition on, wait for the light to go out, then start. On recent diesels it's so quick you barely see it.
Flashing while driving: an engine fault
On many cars, including Volkswagen, Renault and Peugeot, the same light doubles as the diesel engine fault indicator. If it flashes once you're moving, the engine computer has stored a problem and often puts the car into limp mode: limited power, sometimes capped revs.
Most common causes:
- A clogged EGR valve, especially on cars that do short or city trips.
- Boost pressure out of range: split hose, sensor, solenoid valve.
- A sensor: mass airflow, fuel rail pressure, camshaft position.
- Fuel injection, less often.
- A glow plug or its relay, when cold starts get difficult.
What to do
- Drive gently, without hard acceleration, to a garage.
- Switch off and restart once safely stopped: limp mode sometimes clears, but the fault stays in memory.
- Get the fault code read. The code, not the light, tells you which part is at fault.
- Stop if the car smokes heavily, jerks violently or a red light appears.
The short-trip connection
A diesel that only does city driving clogs up: EGR, particulate filter, intake. The same driving pattern that triggers this light often triggers the DPF light. A proper run on a fast road now and then does the engine good.
Keep a record of faults
A light that comes on and disappears after a restart is easy to forget. Autofy keeps every reported fault with its date and mileage, so your mechanic sees the full history. Every other light is in our dashboard warning lights guide.