I have a 2000 s10 four wheeldrive 4.3 the heater

Check to make sure your blend door is functioning properly inside the dash(the door that directs air onto the heater core) If your heater core isn't plugged and thermostat and the rest of your cooling system is working properly then this would be the next thing to check. Sometimes you can hear it moving when you move your temp control if not then you will physically have tp check it. Good luck.