The heater control valve opens and closes to allow warm coolant to flow through the heater core. Air passing through the heater core creates hot air in your vents. It sits between the water pump heater hose and the heater core. The heater control valve is found in front of the firewall of the vehicle.
Remove cover screws and heater core screws. If you are changing the core, once you get it out, carefully remove the foam pad from the old core and wrap the new core with it to avoid rattles.