I would first change the cabin air filter. This is a common cause of this problem.
If that doesn't fix it then you are looking at a problem with the blend doors or actuators under the dash. You can put your head under there when you try to change it to see if you hear them moving or not.

Answered by promechanic (expert)
556 answers provided

Ok, thanks for your help! I will try that!

Response from dboogie
1 question asked

We have a 2005 trailblazer that has been having the same problem. We took it back 2 different times within the first year for the same thing. Amazingly when we took it this time they was able to tell me what the problem was. When they took it to the shop to check it the A/C was working correctly. When they called us to tell us what was wrong with it, they said that when it has a problem like that it saves the code. He said that the actuator needed to be replaced $250. When we got back to pick it up I asked why they couldn't get the code like that when we brought it in for that 6 yrs ago. He couldn't give me an answer.

Answered by paulcobble
0 questions asked