Faulty instrument or temperature sensor?

[Ottar T]

On our A6 w/climate control the 51channel indicates 90deg C while the
indicator on the instrument board stays below 70deg C. This points me to a
faulty instrument or temperature sensor.
Anybody out there with a Service manual who can share information on how to
test the instrument and the temperature sensor?
--
Ottar T
'86 Audi 80 1,8
'97 Audi A6 2,6