[xDrive50i]

Sounds like the temp sensor in the steering wheel is either defective or the wiring between the controller and temp sensor is interrupted (disconnected) somewhere. I'm just guessing that's what the error message means.

I would assume that the controller uses a closed loop type of heat control mechanism meaning that it maintains/controls the temperature of the steering wheel via a temp sensor mounted within the steering wheel (most likely right under the leather close to where you have your hands) for feedback information. It could be as simple as a disconnected wire or harness somewhere or it could also mean that the actual sensor if defective. Have you looked to see if there are any connectors that might be disconnected within the steering column?