I have a 2001 156, the heater fan settings 1,2 & 3 seem to be broken, i.e. they do exactly what setting 0 does, nothing. But setting 4 blows air at its proper presure.
Has this happened to anyone else? What can I do?
I haven't checked the circuit diagram but what is probable is that setting 4 is a direct connection to 12v and settings 1, 2, 3 either go through an electronic PWM controller to regulate the speed or a resistor network.
You need the manual to locate the board that does this, then get it out and see which tracks are burnt or components seem nuked. You can often source them from somewhere like RS and solder them in yourself.
Sorry if it all seems a bit obvious, I don't have a 156 manual to provide any specifics.