Hello my name is Sam, welcome to Just Answer.
The most likely cause of youre issue is a shorted stepper motor bus wire or a faild stepper motor taking the bus down.
To gain access to inspect the entire bus is very labor intensive and since removing the front airbags and removing the dashboard may be nessecary, I would recommend that a dealership perform the diagnosis.
Sam,
I do not understand... What exactly is a stepper motor? Does this term stand for the motor that opens and closes these "doors" to let either hot or cold air pass through? And what is the "bus"? Is it some sort of controler for the different stepper motors/actuators?
And: can the diagnosis only be done in a dealership (closest dealership is 50 miles away)?
Hi again.
Sorry about the nomanclature. Yes, the stepper motors are a set of 5 small electric motors that are connected together with the same 3 wires that stretch from the A/C control unit to all of the motors. These wires are called a bus. Signals are sent over the bus to each motor to tell it what to do. If one motor shorts out, or the wires short out, the communication can become disturbed to all the motors or cause them to move to incorrect positions.
A dealership would be your best bet but an independant BMW specific garage should have a scanner that willl communicate with the AC control pannel.
BMW Master technician
BMW S.T.E.P. graduate, 10 years BMW dealership experience