2001 Buick lesabre

The directional vents (mixing vents) are under the dash ,,, with the engine running you should hear a noise when you change the vent setting. The vent actuators are vacuum controlled through the knob you use to set the direction/function This knob uses both current and vacuum directional ports to send the signals to the actuators ... over the years some of the seals (O rings) become brittle and even crack ,, this causes partial and sometimes no control of the directional vents ... Your problem could be a number of things but I would start at the control knob before going under the dash

I have a 2001 buick lesabre abandoned on my property for 2 months in stockton ca free