Short answer
On a diesel, the coiled glow plug light at start-up is normal: the glow plugs are pre-heating the cylinders, and you should wait for it to go out before cranking. If it stays on or flashes while driving, the engine computer has found a fault, often with reduced power, and the code should be read soon.
- Also shown as:
- Glow plug indicator · Diesel pre-heat light · Coil light
- What it looks like:
- A coiled wire like a spring or a pig's tail, usually amber.
Is it safe to keep driving?
Yes if it is steady after start-up. A flashing glow plug light on many diesels signals an engine fault and reduced power; have it read soon.
What causes the glow plug light (diesel)?
Most common first. Symbols and meanings vary by manufacturer; your owner's manual has the definitive meaning for your model.
- Normal pre-heat. When you switch on the ignition, the light stays on for a few seconds while the plugs heat up; longer in cold weather. That is not a fault.
- Failed glow plug or glow plug relay. One dead plug makes cold starts slow and rough, with white smoke for a moment. A failed relay or control unit stops all of them heating.
- Engine management fault. On many diesels the same light flashes to report other engine faults — fuel pressure, sensors or the particulate filter — sometimes with reduced power.
What should you do right now?
- Wait for the light to go out before starting when cold.
- If the engine is slow to start in the cold or smokes white at start-up, have the glow plugs tested.
- If it flashes while driving, have the fault code read.
What does the fix usually involve?
A shop measures each plug's resistance and the supply from the relay. Plugs are often replaced as a set.
Glow plugs can seize in the cylinder head on some engines, so removal takes care; a technician who knows the engine avoids snapping one.
If the light is flashing for another engine fault, the stored code decides the repair.
How much does it cost to fix?
$80 – $500
A glow plug set or relay.
A typical range, not a quote. Actual cost depends on the car, the shop and the fault.
Common questions
Why does the glow plug light stay on longer in winter?
A cold engine needs more pre-heat before the diesel will ignite, so the system holds the plugs on longer. In warm weather the light may barely show.
Can I drive with a failed glow plug?
Usually yes once the engine is running, since glow plugs mainly help cold starts. Starts will get harder as the weather cools, so have it fixed before winter.
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