Coolant Bypass Valve
WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
The coolant bypass valve controls coolant flow to the auxiliary heater core. Integral to the coolant bypass valve is an electric solenoid that controls vacuum flow to open and close the valve. When the HVAC control module applies 12 volts to the integral solenoid, the solenoid applies vacuum to a diaphragm that closes the water valve. This action restricts coolant flow to the auxiliary heater core. The coolant bypass valve is a normally open valve, if there is a concern with control of the valve or with its vacuum source the valve will still be able to supply heated coolant to the auxiliary heater core.