Right after my Acadia went out of warranty, on a damp rainy day, my windshield began to fog up. I turned on the defroster and had ice cold air blowing out the dash & floor vents, but no air coming out the defroster. This is a safety hazard because I could not see where I was going. The dealer fixed it by charging me $100 to "recalibrate" my HVAC system (whatever that means). A year later the same problem recurred and the dealer recalibrated the system again without charge. That did not help me see where I was going the second time it failed! Now, it seems like whenever we do not use the defroster for an extended period (like, over the summer), we get the cold blast of air out the dash & floor vents but nothing out the defroster. I've found that if I play with the control, going from defrost to vent, back and forth several times, sometimes eventually the defroster works. By the time that works & the window finally begins to clear, we could be wrapped around a tree! Still in search of a permanent fix. In the mean time, I keep a clean rag in the map pocket to wipe the window (just like my grandfather did before cars had defrosters). GMC. We are professional grade!