96 buick regal, w/dual climate control, w/ac on blows hot air on driver side and cold air on the passenger side

Does the vehicle have dual zone climate control, where the driver side and passenger side have different control? Sounds like a blend door issue on the driver side. Why did you replace the passenger side, was it blowing hot too when AC was on?

There should be ( 2 ) slide switches or buttons that control passenger/driver side climate control, one of the switches is not operating correctly and probably be faulty. I have a 1991 Buick Regal with the same problem, sometimes i can slide the faulty one real fast to the right and back to the left and it will work fine.

driver side actuator for blend door is electric servo controlled. It could be jammed or computer sending it in wrong direction. Unit is located on driver side of heater case.

if it is dual zone climate control, then quite possible is the driver's side actuator or blend door.

you need to replace the actuator most likely

Start with making sure it is full of freon. Are both lines coming from and to the evaporator cold?

My 02 Century has dual temp controls . One for driver for passenger. Make sure you don't have the dual climate enabled and just set for heat on your side and AC on the passengers

the motor for the passenger side
blend door is shot

I had the same problem when i was working on my 1999 Buick Regal and had to disconnect the battery. I didn't reconnect in the same way that I disconnected the battery from the terminal and noticed that the driver side had no heat or air but the passenger side did. I then disconnected the battery again and reconnected it the same way and now have heat and air on both the driver and passenger side. Hope this helps. Good Luck!

I would assume with multiple people having the same problems, there should be a service bulletin on this issue. may want to call dealer with vin # to verify. sounds like a heater control issue under the dash. something i would leave to the experts for repair