Glow plugs are used to warm the fuel mixture within the precombustion chamber in a diesel engine. A glow plug is an electric resistance wire within a small housing that is raised to a certain temperature when the engine is started (or "cold"). The fuel plug gets the fuel mixture up to the proper ignition temperature. Once the engine is warmed up, the glow plug control senses the temperature difference and turns off the glow plugs until they are needed again.