A common fault in the BMW climate control system is a resistor that controls the blower motor speed. When this resistor is faulty, your climate control fan speed will do whatever it feels like doing whenever it feels like doing it. High speed, low speed, middle speed or no speed at all regardless of what you set it to indicate a bad final stage resistor. This resistor also has a habbit of running your BMW's battery down and leaving you with a "no-start" condition. It's pretty obvious when it's gone bad and you'll know if you can't control the fan speed accurately.
