This module controls the preheating of the glow plugs in the ignition system. Glow plugs are located similarly to spark plugs on gasoline engines, and have a similar function. Where as a spark plug transmits an electrical charge to ignite fuel in the combustion chamber, the glow plug simply works like a heater to warm the combustion chamber. On days where your engine is too cold to start by itself, electric will flow through these plugs, generating enough heat for them to glow. If this module has failed, starting your engine may be a nightmare when its cold.