What does the squiggly line light mean on a diesel?

It is the glow plug or preheat lamp, and on a cold start it is normal — the plugs are warming the cylinders and the lamp goes out when the engine is ready to start. It only indicates a fault if it stays on with the engine running or flashes.

Can you drive with a glow plug light on?

Yes. The engine runs normally once warm and nothing unsafe is happening. Expect harder cold starting and some white smoke, and get the plugs replaced before one seizes in the head, because extracting a broken glow plug is a much bigger job.

Do glow plugs affect the MOT?

Not directly, but they can cause a failure indirectly. Failed plugs make a cold diesel burn poorly, and both excessive smoke and an emissions test that cannot be completed are major defects.

Dashboard warning light

Glow plug light

Last updated 27 July 2026

Quick answer

On a cold diesel this is normal: the glow plugs are warming the cylinders before the engine will start, and the lamp goes out when they are ready. If it stays on after the engine is running, or flashes, it has become a fault warning instead.

What to do: Book it in, not urgent
Can you drive?: Yes

Why it is amber

A system has reported a malfunction, or something needs attention. The car is usually still driveable, but the fault will not clear itself and several amber lamps are an automatic MOT failure.

What turns it on

Switching on the ignition of a cold diesel energises the glow plugs and lights the lamp for a few seconds. Once combustion chambers are warm enough the lamp goes out. Many manufacturers reuse the same lamp, flashing, to signal a glow plug circuit fault or as a secondary engine warning.

What to do about it

When cold, simply wait for it to go out before turning the key or pressing start. That is the lamp doing its job.
If it flashes or stays lit with the engine running, have the glow plug circuit checked. It is a fault indication at that point, not a preheat signal.
Hard starting on cold mornings with a puff of white smoke is the classic sign of failed glow plugs.
On some cars this lamp doubles as a general engine warning, so it is worth having the codes read rather than assuming it is the plugs.

Can you keep driving?

Yes

A glow plug fault does not stop a warm diesel running and is not dangerous. It makes cold starting progressively harder and rougher, and a plug left to fail completely can break up inside the cylinder head, which is a far more expensive job than replacing it in time.

What causes it

One or more failed glow plugs, which is normal wear on a high-mileage diesel
A failed glow plug relay or control module
Corroded or broken wiring to the plugs
On some models, an unrelated engine fault sharing the same lamp

Does it fail an MOT?

Not on its own — but what causes it often does.

The glow plug lamp is not itself a defect. Where it matters for the MOT is emissions: worn glow plugs make a cold diesel burn badly, and excess smoke or an incomplete emissions test are both major failures.

Fault codes that turn this light on

The lamp tells you which system reported a problem. The code stored behind it names the check that failed, which is the difference between knowing something is wrong and knowing where to look. These are the ones that commonly sit behind this symbol.

P0380Glow Plug/Heater Circuit "A" Malfunction
P0382Glow Plug/Heater Circuit B
P0671Cylinder 1 Glow Plug Circuit
P0672Cylinder 2 Glow Plug Circuit

Got a different code? Look it up — the index covers every standard OBD-II code.

Common questions

What does the squiggly line light mean on a diesel?: It is the glow plug or preheat lamp, and on a cold start it is normal — the plugs are warming the cylinders and the lamp goes out when the engine is ready to start. It only indicates a fault if it stays on with the engine running or flashes.
Can you drive with a glow plug light on?: Yes. The engine runs normally once warm and nothing unsafe is happening. Expect harder cold starting and some white smoke, and get the plugs replaced before one seizes in the head, because extracting a broken glow plug is a much bigger job.
Do glow plugs affect the MOT?: Not directly, but they can cause a failure indirectly. Failed plugs make a cold diesel burn poorly, and both excessive smoke and an emissions test that cannot be completed are major defects.

This is general guidance, not a diagnosis. Manufacturers draw the same system differently and reuse symbols for more than one function, so check the handbook for your own car before acting on anything here.

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