What does the glow plug light mean?
On a diesel engine, a briefly illuminated glow plug light at startup is normal — it means the glow plugs are heating up to aid cold starting. If the light stays on after startup, flashes, or comes on while driving, it indicates a glow plug fault or another diesel-specific issue. A faulty glow plug typically causes hard starting in cold weather and rough running when cold.
Likely causes
Failed glow plug
One or more glow plugs have failed — hard cold starting is the main symptom
Glow plug relay fault
The relay controls all glow plugs — failure affects all of them
Engine management fault (secondary)
On some vehicles the glow plug light is used as a general warning light for other faults
Repairs you may need
Typical cost range: £80–£300 — Glow plug replacement £80–300 depending on how many need replacing and how accessible they are. Older engines carry a risk of plugs snapping during removal.
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