Sometimes a heater fan blower motor will only work on the highest fan speed setting, or not at all. febi provides some information on a possible solution to this problem on some Saab 9-3 and Opel Vectra and Signum models.

Problem
Interior heater fan motor only works at full speed or does not work at all.

Causes
The water drains located below the windscreen are blocked causing water ingress. Cabin filter airflow is restricted. The heater motor is seized. Faulty resistor relay. Faulty heater motor resistor.

Solution
The problem may stem from more than one cause.

If the heater motor is not working and the resistor fuse is blown, check the windscreen drains, and unblock them as necessary. Then, replace the cabin filter and check if the heater motor turns freely, and is free of corrosion before replacing the heater motor resistor.

If the heater fan motor only operates at full speed, check the relay K4 that controls fan speeds1, 2, and 3. This is situated near the heater motor. It is necessary to remove the glovebox in order to access it.

The K4 relay controls the heater motor fan speed settings 1, 2, and 3. Setting 4 is controlled by the K5 relay. The relay acts as a switch to provide power to the relevant fan speed setting. If the K4 relay is no longer working, it will not be possible for the motor to work at the speed settings 1, 2, and 3. Replace parts as necessary.