1993 Buick Lesabre. AIR conditioner

Look for a vacuum problem. All that ductwork is vacuum operated.

-Dan

Yep, most likely vacuum origin.

You may have a bad vacuum module, accessible from inside the passenger compartment, installed in the AC plenum. This controls the transfer of vacuum to the various diverters.

These things are damn expensive.

You may also have a stuck diverter. Sometimes you can free those up manually with a little gizm in a can.

The HVAC computer is always a possibility with GM.

Last, an ex friend of mine had a repeated problem of this type,and the dealership fumbled two HVAC computers into her system. No luck. Finally, they found the problem which was a bad ground connection.

Several possibilities.