Agreed; check your vacuum lines and see if the system is getting a vacuum source to work with. It needs it to change its duct settings.

Check for a hissing sound and a disconnected vacuum line in between the throttle body and the firewall, and around the vacuum tank.

The vacume tube close to the master cylinder if you do not know what that is it is the storage tank for the break fluid just wiggle and push the line in as far as you can of look for it if it fell off i had the same problem.

Yep, mine came loose from same area.