I'm having trouble with the heater. It blows cool air. on 2002 Chevrolet Avalanche 1500

I can only get cool air whin I turn my heat all the way to hot, and turn the blower on. I know that it changes when I change it from dash to floor to defrost and so on but just cool air. Can you help me

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2 answers
The 'mode control motor is responsible for defrost, vent and floorselections. The 'blend door' actuator , or control motor is for temperature , along with a heater control valve. Maybe check those, along with coolant and proper engine temp.
Check to make sure that your coolant level is full and your engine reaches full operating temperature. How old is the thermostat? It probably needs replacing.