Heater / AC I have a Grand Prix and when the.

Hi you have a blend air door sticking possibly just stuck or vacum hose to it leaking. Look under dash watch levers moving while you change vents with car running you will find a problem with a blend door.

If no prob there check vacum at vacum resavoir under the hood

No you don't have to take everything apart. If it is sticking something is prob stuck in it if not just spray a little wd40 on the hibges

The mode door is electric and for temp control only.

The mode selection is done through a series of air doors that are operated by vacuum. When there is either a partial or total loss of vacuum, the system will default to either defrost or on some vehicles, heat. Acceleration can also have an effect on the doors for the same reason.

Look under the hood for a small black plastic hose coming through the firewall. Follow it to the vacuum source looking for any breaks, cracks or leaks. Also follow any other lines that branch off because they may go to a vacuum reservoir (storage tank) and a leak there will cause the same problem. Using a vacuum gauge to determine if you have full vacuum at any given point is the best way to find it. You can cut into the line anywhere and splice it back together with a piece of vacuum hose. Find the broken line, you found your problem.