My '98 Buick’s AC works well when I take it to a mechanic. But when I leave after they say, ‘it seems fine now,’ a few miles later the cold air starts blowing through the defroster vents, still nice and cold. Then it comes back down to the dash and then goes back up to the defrost. But sometimes it stops blowing altogether, it doesn’t blow anywhere! Nothing, not even warm air. And then later everything is fine again, until it happens all over again and I take it to another shop when I hear ‘it seems fine now.’.
The first thing I would suspect is a defective Vent Control Module. The Vent Control Module is an electronic controller for the ventilaton system. You see all the contols for the ventilation system? That’s the Vent Control Module.
Do you have the automatic climate control on your Buick… I also have a 98 Lesabre so I have some experience with that unit.