There were various posts online suggesting that cleaning the resistor pack with Vinegar could fix the problem. It seemed somewhat bizarre but with nothing to lose I bought a bottle of vinegar, found a suitable dish and left the resistor to fizz for a couple of hours. I then washed it off, dried it and left overnight to fully dry out.
After refitting under the bonnet I tried the heater fan again - magically it now works on all 4 settings! A very cheap and easy fix!
|Porsche 944 Fan Runs at Speed 4 only - Resistor Cleaning with Vinegar|