Regarding a 2002 Sierra 2500 - When the a/c is on sometimes it will just blow hot air. Any suggestions would be appreciated.

It could be a few things. First try to see if all of the vacuum lines are connected. It there is a leak the system will not work properly. It could also be the heater switch causing the problem.

Behind the glove box

Did you bleed the air out?

Im having a similar problem, sometimes my blower works other days it wont will when it does work no hot air comes out, blower controls change but no hot air, i replaced the thermostat and now my blowers are not working!

Suspect temperature control servo If stripping noise is heard when changing from hot to cold and back,replace servo.

Vacuum. Your brakes probably work off of it too so it feeds all of the vacuum to the brakes and pulls it away from the A/C. I had an S-10 Blazer that did that.

i have a 04 sierra 2500 hd, i put a spectre cold air intake and it boosted my economy to 11 city 16 hway and it was only 150 at autozone

Probably your left temp actuator failed internally. Check the right instrument panel junction block-right side, end of dash-to make sure connectors are fully seated. But thisis usually caused by a faulty temp actuator.

THERMOSTAT REPLACE IF NOT SEE IF HOSES ARE CLOGGED SHOT OF AIR COULD HELP OR DISSCONNECT BATTERY CABLE 10-15 MIN CONNECT BACK TRY HEATER

This sounds like the mode selector is stuck on heat. Under the dash is the location of the hvac plenum and you need to check the cables and levers that are on it to see if they move when heat and cold are changed as well as the lever for where the air is directed.