2006 envoy Denali a/c was working fine and now only blowing from the side vents

I experienced the same with my 2006 Envoy.Look up fuse and relay numbers fuse location is in engine compartment . My problem was the relay. There are 2+ relays with the same number. I removed relay and replaced with one of the existing relays and A C worked. I replaced original relay and problem returned. Don't know why,I switched the same two relays and both Ac and other circuit working. Zero $$$$$ fix . Note: I wrote the other circuit on the fuse page. Hope this work for you.