Glow Plug

Diesels rely on compression heat to ignite fuel. In cold weather the cylinder walls absorb too much heat, so glow plugs warm the combustion chamber before and just after starting. Modern plugs heat up within seconds. Many engines keep the plugs on after starting (post-glow) to reduce smoke and noise. A glow plug control unit drives them and reports faults.

Modern glow plugs heat up in a few seconds, and many are made of ceramic. The control unit also keeps them working after starting to reduce noise and smoke during warm-up.

The plug’s tip contains a heating element. The control unit switches it on according to coolant temperature, and the glow plug light on the dash shows when to start. A glow plug control unit switches them on and monitors each plug; a fault is stored if one does not draw the expected current.

Failed glow plugs cause hard cold starts, rough running and white smoke after starting, and codes in the P0380 range. Removing seized plugs can be delicate on some engines. Faulty glow plugs cause hard cold starting, rough running and white smoke after starting. Removing old plugs can be difficult because they seize, so this work is often best left to a workshop.