ACDelco GM Original Equipment Heater Bypass Valves are control valves for the flow of coolant through the heater core. When the heater valve is closed, the bypass valve diverts the coolant flow back to the engine. The valve maintains coolant flow through the thermostat housing to ensure correct operation of the thermostat. This original equipment heater bypass valve is a GM-recommended replacement for your vehicle’s original components and has been manufactured to fit your GM vehicle, providing the same performance, durability, and service life you expect from General Motors.