exterior temperature problem

My exterior temp reading has gone haywire. Has varied from -40! to nearly correct, and goes up and down like a yoyo!
Audi have replaced the sensor, under the front bumper (twice) and then suggested a new dashboard instrument cluster. Got one from david (phew), but it made no difference.
Audi then suggested the climate control unit. Again, got one from David, but it doesn't seem to have improved things, with the exception that the temp doesn't go up & down to such extremes.
It almost seems like it needs calibrating, but how?
Audi are adamant that it is not a wiring fault.

Apparently -40 is a default value if the dash unit does not understand the information it is being sent.
Has anyone got any bright ideas?