I have a 2004 GMC Pick Up and the A/C changes to heat when I’m driving down the highway. If I stop, turn off the engine and re-start it the A/C is OK again. This happens periodically and without warning. What could be the problem? My dealer is baffled.
Since I doubt if it is still under warranty I would suggest taking it to a A/C shop, not the dealer. There are a number of possible issues and a good A/C shop is most likely to find it.
if your dealer is baffeld at that common issue (which I doubt) find a new dealer.