I've been having this odd issue with my S4 air con. Basically, sometimes it puts the fans to max and blows hot air. When the air con does work it blows nice and cold so I don't think it needs regassing. I've tried turning it on and off again, I can hear a difference in engine note when turning on and off the Econ mode. The car has been serviced and no fault codes show up. Hard accelleration at high speed gets it working again and it will then work for the rest of the journey.
I've taken it in to Audi who says it needs a new compressor at a cost of ££££. I'd like to rule out everything else before going this route.
Full history of this on Pistonheads: B6 S4 Air Con Issue S4 Air Con Issue