Why would a 2001 Pontiac Grand Prix GT heat work on a high setting It was on the 3rd setting and now I have to put it on the 4th setting in order for it to work?

fan selector switches are usually riostats it will not make good contact on other settings and therefor only works on 1 setting change switch byou can pull it and check with meter

Replace the blower resistor. It is under the glove box, behind the blower motor.

My 2001 Grand Prix did the same thing. It used to work nicely on setting 2, and over time had to increase to setting 3, 4 and so on to get the same amount of heat. Eventually was getting no heat at all. Brought it to the garage and they replaced the blower mechanism and all was well.