Diesel glow plug light: what it means

On diesel engines, the glow plug light (orange or yellow spiral) indicates that the glow plugs are heating the combustion chamber before starting. Understanding how it behaves helps avoid misinterpretation and breakdowns.

Normal behaviour

When you turn the ignition on, the light comes on for a few seconds (often 2–5) while the glow plugs heat up. When it goes out, the engine is ready to start. In cold weather, the light may stay on longer. This is normal.

When to worry

Light stays on after starting or never goes out: fault in the glow plug circuit (plugs, relay, fuse). Light flashing: often a fault stored by the ECU (glow plugs or other). Very hard starting when cold despite the light: worn or faulty glow plugs.

Common causes

Glow plugs worn or burnt out (one or more). Glow plug relay or fuse faulty. Wiring or connectors corroded or broken. On some vehicles, a faulty temperature sensor can keep the light on.

What to do

If the light stays on or flashes, have the glow plug circuit checked (electronic diagnostic, resistance check on plugs). For hard starting when cold, replacing the glow plugs (often as a set) usually restores normal starting. My Auto Check-up helps you prepare your garage visit for a cost estimate.