My 2000 GMC Jimmy has been blowing cold air. We have changed the thermostat and flushed the radiator and heater core. The air is not as cold but is not warm. What do we do next?

well either your heater core lines are clogged ore the heater core is just bad

common for this vehicle to have clogged heater core

I've had this problem. 2000 GMC uses a closed coolant system. Meaning takes and returns coolant from and to the Reserve Bottle and has little air in the engine and heater systems.
If everything appears ok but no heat. "Air Lock" is the most likely cause. (With interior heater set to full heat) Solution; 1. Make sure water pump works. 2. Drain about 1/2 engine coolant into container. 3 Remove both heater hoses from engine side and flush heater core in both directs through heater hoses with garden hose. 4. Reattach heater hoses in correct locations. 4. Refill engine with coolant. 5 Start engine and run for 2 min., use caution when topping up coolant.
6. Most 2000 GMC have twist off caps with plugs located mid way on heater hoses. with engine cold, start engine these caps may be loosened to allow trapped air to escape. Do not attempt with engine hot due to hot coolant injuries. This may take more than one attempt to remove all air from heat system. 7. Lastly most 2000 GMC water pumps have a bleeder plug,( 3/8" square driver plug) located on the water pump. With egine cold this plug should be removed to remove any air. Reinstall plug with thread sealant.
Hope this helps cheers

UNHOOK BOTH HEATER HOSES AND FLUSH OUT HEATER CORE
DO IT FOWARD AND BACK FLUSH / COVER DIST. CAP OR YOU WILL
FILL IT WITH WATER / IF IT CLEARS OUT HOOK HOSES BACK UP AND
BLEED AIR OUT OF SYSTEM / IF IT DOES'T CLEAR OUT HEATER CORE
WILL NEED TO BE REPLACED (DASH COMES OUT TO DO HEATER CORE)

Mine did the same thing you need to have heater core flushed out. Dexcool coolant clogs it up get it and the radiator back flushed and changed to regular coolant. This should fix the problem.

my 2000 saturn sl blows cold air with the heater on. I changed the thermostat,and checked out my heater core.Still blows cold air.

once truck is warm check both heater core hoses to make sure they are both warm if not then check the cold water valve could be stuck shut not allowing coolant to travel in to heater core.

Flushing the heater core is fairly easy just remove the 2 hoses going to the heater core and use a different hose if you have to and use a garden hose to backflush the blockage out..this may take as much as 5-6 times also check for vacumn lines missing or disconnected or breaks pinched ect... some where in the vicinity of the passenger side under hood by the a/c lines ect. good luck hope this helps

I had problems with a similar truck,a 1998 GMC diesel after checking to make sure the control cables were operating the blend door and blowing the mess of the heater core,I replace the thermostats.Still no heat turns out there was a small amount of debris collecting in the fitting for the inlet hose at the thermostat housing after removing it and blowing out the lines the heat was fine.

Had similar problem in my 2000 Jimmy. No hot air from heater. Had the shop (Midas) disconnect heater hoses at the fire wall and blow out the heater core with air hose. Got rust and junk blown out. Hooked up and flushed system with new anti freeze. Got lots of heat now.