2000 gmc siera with heater problems

i recently changed my themostat because i was having problems with cool air blowing out my vents when i wanted hot air, the engine heat stayed consistantly at aprx.190 degrees, and the fan worked fine, but when i would move my temp.to the hot side it would only blow out cool air ,i also checked my antifreeze level, it seemed to be fine, what else could it possible be? would appreciate some professional advice , thx

With the car warmed up touch both hoses going to the heater core. They should both be hot, about the same temp. If one is noticably hotter then the other you may have a blocked heater core. The hot hose will be the "in" and the cooler hose will be the "out". remove both hoses and back-flush (hose to the out) the heater core until the water comes out the "in" line as clean and as fast as it enters the "out" line.

may also be a damper stuck in the ductwork , mine gets stuck on HOT sometimes

I have a 04 gmc Sierra w/similar problem, what i have learned so far is the air door actuator is responsible for directing air flow thru the heater core . It's located close to the floor in the middle of the floor hump. I suggest that you find the thing, remove the 3 screws that hold him in his job and then move the tempertature control from max to min & see if it rotates . If not , that is the "Culprit "

I have a 01 Sierra and it gets stuck on hot. I changed the heater control module, and that did not work so it has to be the damper.