Checking the diesel preheating device (Mercedes-Benz C-Class W202)

If the preheating indicator lamp does not flash although the ignition and instrument switch with the anti-theft device that locks the steering wheel is in the preheating position, then the preheating duration of the diesel combustion chamber is determined depending on the outside air temperature and the engine starts. If the engine starts well, check the glow plug glow lamp and electric line. If the engine cannot start, check the preheater armature current circuit for an open circuit. If the engine does not start well, check the glow plugs. Check the temperature sensor if the preheating duration is too short or too long.

Caution! If the engine is difficult to start, although the glow plug indicator light indicates that the preheater is functioning properly, one of the glow plugs for cylinders 2–4 may be faulty. In this case, also check the glow plugs.

Remove the cover from the heating control relay. To do this, on the input side, throw the wire under the cover and tilt it up. The heating control relay is located in the engine compartment on the left wing.