I've had a problem for the last year or so with a random mis that eventually sets a code on the computer. I finally tracked it down to the air conditioner, of all things. What is happening is if I have the AC on and stop in traffic, the head pressure goes sky high because the fans don't go on high like they are supposed to do and it drags the engine down. The fans will come on when the car eventually heats up. The pressure sensor works, I can see its voltage rise on the Autotap scanner with pressure increase. The service manual says that fans are controlled by a combination of temperature and compressor head pressure. Obviously, head pressure isn't turning on my fans. Any ideas on how to fix this, I'm stumped.

 

Sounds like your THX valve is bad. Can't understand why the sensor works but the PCM doesn't react the way it should though. Have you done a search in GM data base to see if this is a continual problem that has a service bulletin?