It could be as simple as a problem with the thermistor in the driver's side wing mirror that supplies the temperature to the display unit. Unfortunately I can't remember what this is, but you could check the resistance using a variable resistor to get the resitance for 25degrees C, then order the part from maplin or somewhere for a few pence, then install the new one... :bang:
or Fiat will sell you a new one for about 10 quid (which is probably easier than the above.....
saves a lot of hassle that way....
If it is not that, then it is an aircon controller issue....