have a 2006 a6 SE and for some reason the air con wont turn off. Whats happening is as you drive along you have nice warm air and full control over the fan speed. After a minute or so off driving if you look into the engine the air con pipes are starting to go all frosty. Econ mode is selected but doesn't seem to do anything. You can hear the compressor starting when you turn off Econ but that is it.
After 10mins or so of driving no air comes out of the vents regardless of what you do. You can hear the fans going in the engine bay but no air comes into the car. But when the car stops these pipes start to defrost and the air comes back out again until you start driving. We cut the wire going to the air compressor and this solved it but need to get it fixed.

 

Never hear of an A6 SE. But you need to hold to the ECON button and that will shut the AIR OFF.
It sound like the like compressor valve is sticking, here a link to the fix: HVAC Blows very hot air on side vents - Page 3 - AudiWorld Forums