Hi, I've been looking through old posts and a few sound like my problem, but I have no idea of where to start looking (I don't know that much about this system). I have a 1999 Blazer with the computer climate control and all of the settings work, except the temperature dial. It usually only puts out heat in the full hot position, but every once in awhile 78-82 puts out warm air but not as warm as it should be. Also, the fan seems to be making odd noises on occasion. Sorry for the long post, but I have no idea where to begin. I thought it was the system's brain couldn't tell the correct temp anymore.
Any help is greatly appreciated!
Thanks

 

Have you checked the heater core? It may be plugged.

 

I can't help with the climate control but if it is low on coolant it will not put out heat. Don't just check the coolant tank. When it is cool take the radiator cap off and make sure the radiator is full.