I have a friend with a 3 year old Audi A6 (2.7 diesel I think) that has had
a problem for a long time now with the aircon intermitantly going a bit
haywire and either blowing very hot or very cold.

Garage is at a loss as nothing shows up in the computer logs (and I guess
intellegent guesswork/diagnosis is beyond them) and because the problem is
only intermittant and typically never happens when it's taken to the dealer.

Are there any common known problems with this car's aircon?

TIA

Tim