The purpose of the Heater Control Valve is to regulate the flow of coolant to the heater core. In the off position the heater valve blocks coolant flow to the heater core hence when no heating is required and the reverse when heating inside the vehicle is needed. The heater valve can be activated by vacuum control in late model vehicles and cable in earlier models.
Common failures include leakage, blocking and because most late model valves are made of polymer, cracking when removing heater hoses is common. Dayco have released a quality aftermarket program targeted at local and imported vehicles.