Each cylinder will have its own glow plug, controlled by a glow plug relay (or by the vehicle's body computer module (BCM). When the ignition is initially switched on, a current is automatically sent to the relay to switch the glow plugs on.

 

When the plugs are going through this cycle, normally the dashboard will show the pre-heat plug warning, which looks like a coil of wire. Once the glow plugs have heated up, the light will extinguish. Ideally, the engine should only be started once this light has gone out to provide their maximum effect.