I have a 2000 Grand Prix. It has a full charge of Freon and the right side vent blows cold air (as it should) when the AC is on, but the center vents blow a little warmer, and the left side vent blows warmer yet. The car does not have dual-zone air. Any idea what would cause this?
Cooler and warmer air coming from the left, right and center ducts may be the result of an inadequate level of performance from the air conditioner's refrigerant system. You stated that the system has a full charge of Freon. But the A/C on an '02 Grand Prix uses R-134A refrigerant, not Freon, so maybe you should double-check.
Here's a brief history of Freon. It was actually a Dupont trade name for a family of synthetic chemical refrigerants, commonly referred to as Freon, Freon-12 or R-12. Freon's harmful effect on the earth's ozone layer was determined by the EPA in the late 1980s, and its use in automotive production was eliminated by the 1995 model year. Its replacement was the less harmful and perhaps not quite as efficient R-134A.
With that said, the first step is to be absolutely sure that the system is fully charged, and the best way to do that is by evacuating and recharging the system with the specified amount of refrigerant, by means of the proper recycling equipment.
Here's where you have to be careful. General Motors had some A/C performance issues on a number of vehicles, including your Grand Prix if it falls within a certain VIN range. The problem was an insufficient refrigerant charge from the factory, and the incorrect capacity printed on the label under the hood. The correct amount should be 2.25 lbs (1.02kg). If you installed less than that, add the difference.
Aside from that correction, the system needs to be evaluated by a trained technician. System refrigerant pressures have to be tested, and manual and/or electronic temperature controls should be inspected for proper operation.