Sounds like the Heater exchange valve. I had the same problem the heater on the passenger side worked but the drivers side was not getting warm...A useful web site for you to try and get a new part is british car parts.co.uk
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Low coolant, if there is an air pocket in the engine it will make it to the heater core. Once there , the air coming out is cold/cool. Once the coolant fills the heater core, it gets hot. If the coolant is fine hot or cold, then have the HVAC system scanned.
Check to see if your heater hoses are hot when engine is at normal operating temp. (180 to 220 degrees F) If they don't get hot like the large radiater hoses do, then your heater hoses are clogged, probably at the point they connect to engine. Remove heater hoses from engine and I bet you will see dirt, grime, and corrosion where you took them off, restricting the flow of coolent through your 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
this sounds normal in very cold conditions! The air is never hot when you first start the car in any car, until the engine warms up, then the heater air will also warm. You could check your anti-freeze level for the first thing. If you don't have enough anti-freeze, it won't run through your heater to give you the proper heat
first check to see that you coolant level is full, check it after the engine has cooled down,so there is no pressure in the system. otherwise you possibly have a head gasket in the early stage of failure