Outside Temperature mis-reading
 

My external temperature reading in the central binnacle display consistently understates the external temperature, typically by about 10 Centrigrade, which is a big error.
As an example, yesterday the temperature was 43 Centrigrade (110 Fahrenheit) (as announced on the radio and as indicated on my other cars) but the 928 display indicates 33 Centigrade (91F).
Living in Dubai we are conscious of high tempeartures and I would like to fix these mis-readings. The other digital display info appears correct.
Any thoughts?
I believe the sensor is in the front bumper and there is no reason why it should under-indicate. Could it just be dirty? Is this a small part which is easily replaced?
Thank you
1995 928 GTS Manual