The hot water bypass valve is normally an open valve, which closes when vacuum is applied to the valve. When the MAX A/C mode is selected, vacuum from the HVAC control assembly is applied to the hot water bypass valve.

The vacuum must be strong enough to overcome the tension of the valve's internal return spring in order to close the hot water bypass valve. The return spring forces the valve to return to the open position, when any of the other HVAC modes are selected.

In the closed position, the flow of coolant to the heater core is bypassed, allowing maximum cooling to the passenger compartment.

The hot water bypass valve blocks the flow of coolant through the front heater core, only on vehicles equipped with auxiliary heating.