Re: Which sensor is it?
The external one displays on the infocentre. Usually when they fail they read, I think, -20 all the time, maybe yours needs a clean or has some loose wiring.
As to replacing the sensor, not sure if it's available as a separate part, think someone said that you can only buy the whole mirror housing with the sensor fitted but that could be wrong. If it's not then maybe a trip to the breakers looking for a good sensor would be best rather than fitting a whole new mirror housing.