Have a 1998 buick century when either the air conditioner or heater is turned on cold air comes out of drivers side and hot air n passenger side

SOURCE: 1998 buick park avenue drivers side blows cold, passenger side hot

sounds like the drivers temp door actuator is not working should be located on the drivers side of the air box down by the gas pedal or center dash area.

SOURCE: Only cold air on passenger side

I would first check and make sure that you have enough Freon in the system, on climate control dual systems only one side will put out cold air if the system is low on Freon. Eventually, both sides will put out warm/hot air if the system continues to loose Freon. As with all leaks the system should be checked and then the leak causing component replaced/repaired.

SOURCE: 1998 Buick Century A/C driver's side warm air, passenger cooler

had the same problem but reversed (pass side warmer). It turns out the dual zone climate control unit was broke. Had to have a new one installed. Works fine now.

SOURCE: Heat on drivers side cold air on passenger side.

you just have to recharge you airconditioning unit

SOURCE: a/c blows hot air on drivers sde

The actuator door on the driver's side is out of position and needs to be reset. Remove the negative battery cable for 1/2 hour to reset the computer. Start the HVAC system and you should be able to hear the door adjusting itself and cold should come out of the driver's side vent.