Got this from Silver6 on FMOC

If your Climate Control is blowing cool air when you have it set to hot, or it blows like a neuclear furnace on the coldest setting! it may just need a reset to clear the errors the unit sometimes accumulates. Doing this before other fault finding measures are undertaken can save time & costs nothing! it only takes a minute to complete so it has to be worth a try.

Follow the instructions to the letter & try a few times if it doesn't appear to work the first couple of times.

Step 1: On the CC module, briefly press the "Off" and "Footwell" buttons together, then press "Auto". The self-diagnosis which then starts lasts a few seconds. An animated display appears in the CC display during this time and flaps can be heard operating. Any faults found are displayed in the form of trouble codes. [Example: First of all, "93" flashes for 2 seconds, then "42" flashes for 2 seconds - DTC 9342.] If no faults are stored, then all of the segments in the display are actuated.

Step 2: Press the "Screen" button to erase any fault codes, reset the unit & exit diagnostics mode.

This works for late Mk2's with EATC climate (as opposed to SATC) & early Mk3's! but I can't be certain that this will work for all post-facelift mk3's.