Diesel: DPF, AdBlue and glow plugs
Glow plug light on or flashing: what to do
On a diesel, the glow plug light (a coil) comes on for a few seconds when you switch the ignition on while the glow plugs heat up, and then goes out. If it flashes or stays on while driving, on many cars it warns of a fault in the preheating or the engine management.
Check without hurryDiesel: DPF, AdBlue and glow plugs
Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. If it only comes on at start-up and goes out, that is normal. If it flashes while driving, have it checked soon and avoid long trips; if you also lose power or another light comes on, drive gently to the garage.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Blown glow plugs: one or more stop heating; starting is hard in the cold and there is white smoke.
- Glow plug control unit or relay: it does not power the glow plugs or reports a fault.
- Wiring: a corroded connector or a damaged cable at the glow plugs.
- Other engine faults: on many diesels this light also warns of fuel pressure or sensor faults.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: Why does my car go into limp mode when it is cold? What does a pending, stored or permanent code mean? How can you tell if the 12 V battery is failing?
Car system: Ignition
What you can check yourself
Notice whether the light only comes on with the ignition or also while driving, and whether it flashes.
Wait for it to go out before starting, especially in the cold.
See whether starting is harder in the cold or white smoke comes out on start-up.
Read the fault codes: they show the specific glow plug or circuit.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
- P0380Glow plug/heater, circuit A: malfunction
- P0381Glow plug/heater indicator, control circuit open
- P0382Glow plug/heater, circuit B: malfunction
- P0383Glow plug control module 1, control circuit: signal low
- P0670Glow plug control module 1, control circuit open
- P0671Cylinder 1 glow plug, circuit open
- P0683Communication circuit between glow plug control module 1 and PCM: malfunction
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: preheating status, coolant temperature, system voltage while cranking and fuel pressure.
Frequently asked questions
Why does the glow plug light flash?
On many diesels flashing means the control unit has detected a fault, which is not always the glow plugs: read the code.
Must all the glow plugs be replaced?
Not always, but it is common to replace them together if they are the same age. The code and a measurement show which one has failed.
Does it start normally in summer with a broken glow plug?
Often yes, because they are barely needed when warm. The fault shows in the cold.
Keep reading
- Engine ignition: how it works and what fails
- Car slow to start: causes and what to do
- White smoke from the exhaust: what it means and what to do
- Car loses power: what limp mode is
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means