What is a Heater Control Valve all about?
The heater control valve is responsible for warming the interior compartment. This valve controls the flow of coolant from the engine to the heater core. Most heater control valves are operated in three basic ways. A manual cable, a thermostat type system, or even engine vacuum can control them.

How it's done:
Pressure check the cooling system.
Test operation of the valve .
Replace the control valve if bad/leaking.
Fill with coolant.
Test operation of the heater.

Failure Symptoms
Coolant is leaking.
Heater does not work.