MY 2000 GMC SIERRA HEAT BLOWS WARM BUT NOT HOT SUPPLY HOSE IS HOT BUT RETURN HOSE IS JUST WARM. WHAT COULD IT BE

Behind the glove box

lines not in the correct ports

low pressure hoses are clamped
high pressure are screwed in some how there is a cross porting causing the lock up problem and that is putting high pressure into the low pressure port

losing any fluid...fogging windows, I hate to say it might be your heater core clogged or small leak

Have you tried backflushing the heater core?

I removed the hoses and took a air nozzle at low pressure and blew in the return hose connection of the core. A big chunk of paste like substance shot out then pure liquid. I put the hoses back on and tested it. Now its blowing hot!!!!!!!

Check coolant level. Check for heat at the heater hose going from water pump to heater core at firewall. Check to see that cable is moving coolant control valve from the closed to open position. Low mileage so it shouldn't be plugged heater core.

THE 1ST THING U DO IS GO UNDER THE HOOD WHEN ENGINE IS HOT, CHECK THAT BOTH HEATER HOSES (INLET AND OUTLET) ARE GETTING HOT, IF ONLY ONE IS HOT FLUSH THE CORE, IF BOTH ARE JUST WARM THEN THE THERMOSTAT IS DEFECTIVE IN THE ENGINE, REPLACE IT, LET ME KLNOW WHAT YOU FIND.

Check in the one heater hose will be a control valve. Check the operation of the valve(this is the heater control valve). If the lever on it appears to be moving. Take the valve out and manually check that the valve is opening and closing the whole way. If not replace the valve and recheck. If it is the heater core may be clogged not allowing the hot coolant to flow thru so you can get heat.

sounds like a blend door problem gm uses plastic clips to hold actuator rods that tend to break easiest thing to do is take it to a shop that offers free heater checks. and let them check it out

I experienced the same head ache. First attempt was to replace the thrmostat and after taking it apart I realized that there was nothing wrong with it. I then removed the plastic cover/panel located under the dash board on the passenger's side. (only a few 4-5 smalls screws) Removal will enable you to see the on/off switch/gate moving/rotating as you turn the knob to hot or cold positions, on the control panel. If that switch/gate seems to work ok, then the trouble is within the heater core. Many mechanincs suggested that I replace the heater core. A very expensive proposition.
I checked the heater core supply/intake hose and found it hot to the touch, while the return hose was nearly cold. It was definitely a heater core blockage. I removed both ends (supply and return) hose connections/clamps. They are located near the fire wall in the engine compartment. A special tool is very helpful to remove those clamps easily and to replace them as well. It can be done the old fashion way but this tool was a life saver and reduced work time to just minutes. (Tool is $70.00 bucks at auto parts store,so I borrowed mine from a mechanic/friend)
I then hooked up a garden hose to the return end of the heater core and reversed the flow of coolant/water through the heater core. My truck is a Sierra 2000 and I refuse to see how a heater core could get clogged up with clear coolant alone. To my surprise a flat piece of rubber about the size of a quarter blew out of it and the flow of water became unobstructed. I reconected the ends of the hoses to the heater core, replaced the lost antifreeze & everything returned back to normal. No money spent for something most mechanics suggested I replace control valves/switches ($280.00) replace heater core ($500.00) etc etc. I am glad I was curageous enough to tackle this myself and save a few pennies. Good luck, and I hope this will work for you.