Motronic control unit (computer) has a built-in self-diagnostic system that detects malfunctions of the system sensors and registers them in memory as error codes. However, the procedures given in Chapters Supply system and Ignition system can be used to individually check the components and sensors of the Motronic system. If this fails to find the cause of the fault, then the vehicle should be sent to a BMW dealer who has the necessary diagnostic equipment to read the error codes from the control unit.
You then have the choice of either doing the repair yourself or leaving the repair to your BMW dealer.