I have a 1998 Buick Park Ave Ultra. HVAC climate control. A/C is cold but only discharges at the defroster and floor registers. No matter what position I select, Auto, or driver selected it still won't discharge through the dash registers. There has been a couple of times that when I first start the car that the air will come out of the dash registers but after about a minute it changes and goes back to the defroster and floor registers. I had one incident today where going down the road that it started coming out of the dash registers for just a few seconds then went back to the defroster and floor registers yet once again. I'm wondering if this could be caused from one of the actuator doors not positioning properly and if so which one could it be? Any help would be great. I do know that the actuator door don't work by vacuum but by a DC motor, so any remarks about checking the vacuum wound not help. Thanks

Are you absolutely certain your system is not vacuum driven? Bonneville is vacuum driven up to MY 2000.

Air out the floor and defrost with no change when you use the controls to direct air throught the vents is the default mode when the the HVAC control unit has failed. You probably need a "new" HVAC control unit. You can pick up a used one for a fairly reasonable price.

Check the vacuum thing first though.

My car is a 1998 Buick Park Ave and the electronic HVAC, climate control, dual zone A/C has all of the doors operated by a electrically activated DC actuator. Thank you for the info on the replacement of the HVAC control unit. I was not aware that air out the floor and defrost with no change when you use the controls to direct air throught the vents is the default mode. Once again thank you!