What are Glow Plugs?
The small heaters that help a diesel engine start when it's cold.
Simplified animation — not to scale.
In plain English
Glow plugs are small electric heating elements screwed into the cylinder head of a diesel engine. They pre-heat the combustion chamber so the diesel will ignite reliably on a cold morning, and stay on briefly after starting to keep things smooth.
"Like preheating the oven before the dough goes in — without it, a cold diesel just won't catch."
How it works
When you turn the key, the glow plug controller sends current through each plug's element, which glows red-hot in a fraction of a second. The hot tip warms the air inside the cylinder enough for the injected diesel to ignite under compression. Once the engine's running, the plugs cycle off.
Signs of trouble
- ⚠Hard starting in cold weather, often with extra cranking
- ⚠White or grey smoke from the exhaust on a cold start
- ⚠Rough running for the first minute after starting
- ⚠Glow plug or engine warning light on the dashboard
- ⚠A diagnostic code reporting one or more failed glow plugs
£100 – £400 fitted for a set
Always get a written quote. Prices vary by car, region, and parts brand.