I've got a 1997 Lumina Sedan with nearly 120,000 mis. on it and the heating ducts are stuck on window defrost. I can't change the air flow to the floor or vents...would this problem be in switch itself? By the way the rear defrost portion of the switch does work..Any thoughts?

 

check for broken vacuum lines under the hood. The switch is controlled by vacuum. A loss of vacuum makes the system default to defrost for safety

 

Wouldn't that make the car run like sh@#@$%?

 

Thanks SHAWNVW for the amazing diagnosis! After an indepth search under the hood, I discovered the vacuum line running through the firewall was separated at a union. Hooked it back together and everything works great now!! Great job SHAWNVW!