Diesel engines rely on the heat of compression to initiate combustion. Cold engine start-ups may require extra engine cranking time to create the necessary heat to ignite the diesel fuel. One of the devices available to aid in cold starting the diesel engine are glow plugs. Six glow plugs are used to preheat the combustion chambers as an aid to starting .

This circuit consists of the following components:

When the ignition switch is turned ON, the glow plug relay is closed by the ECM to start preheating the engine. After starting, the glow plug relay remains closed for a calculated time based on coolant and air temperature. After the calculated time ends, the ECM opens the glow plug relay, deenergizing the glow plugs.

The ECM also has a self diagnostic check for the glow plug system. If a glow plug system failure is present, the ECM flashes the wait-to-start lamp as an indicator of the condition.

To initiate the glow plug operation, these starting procedures should be followed:

1. Turn the engine control switch to the run position. Wait until the wait to start lamp turns OFF.
2. Turn the engine control switch to the Start position, and release the switch when the engine starts.
3. If the engine does not start after 15 seconds of cranking, repeat steps 1 and 2 until the engine starts.

When the ignition switch is turned to the ON position the glow plug relay is commanded closed. Current flows through the glow plug relay, and to the glow plugs, heating the glow plugs to 800°C (1,472°F). When the ambient temperature is about 0°C (32°F), the glow plugs will take more than 10 seconds to heat.