2002 Tahoe The air conditioning vents on the dash have stopped
working... The defroster and floor vents still work. The air that is
supposed to be coming out the dash vents is now coming out of the floor
vent.

>defroster and floor vents still work.

Vacuum leak in the door motor. The def/flr vents still work because
those are on a separate 2-stage vacuum motor. When a leak occurs in the
vacuum system, the def vent is the 'default' position. Check for a bad
vacuum hose connection/cracked hose/faulty motor.

Dave S(Texas)