Mercedes-Benz W203 CDI electric auxiliary heater
Mercedes-Benz W203 CDI: where is the electric auxiliary heater and what does it do?
The auxiliary heater is on the side of the engine under the intake manifold above the starter motor, a small aluminium box with wiring. It is usually visible only from below with the starter removed. It switches on when starting a cold engine and heats the cabin, switching off at about 60 degrees.
Mercedes-Benz W203 CDI: how do you set the auxiliary heater operation?
The setting is made with the steering wheel buttons from the trip computer menu, under HEATER. The options are automatic mode (AUTO) and off (OFF).
Mercedes-Benz W203 CDI: what does fault code 2509 mean?
Code 2509 (heater booster) indicates that the auxiliary heater's control signal or position information does not match. The heater may not be in the car at all, in which case the control unit simply assumes it is there. The fault can also be in the heater itself.
Mercedes-Benz W203 CDI: what symptoms can a faulty auxiliary heater cause?
The heater can stay on continuously, which drains the battery. It draws a lot of current, so when on it loads the alternator and can make the idle uneven. The heater is controlled by a relay.