By-Pass Air Control (BAC) Valve
WARNING: This page does not describe the selected car, but rather 9 other vehicles, including the 2003 Mazda Tribute, 2003 Mazda Protege5, 2003 Mazda Protege, 2003 Mazda MX-5 Miata, and 2003 Mazda MPV. However, it is still accessible from the selected car via links, so may be relevant.
The BAC valve contains an air valve and Idle Air Control (IAC) valve. Engine coolant is directed around the air valve, warming the thermowax element. When engine coolant temperature is less than 122°F (50°C), the wax is contracted, opening the air valve, enabling fast idle speed. When coolant temperature is more than 122°F (50°C), the wax is fully expanded, closing the valve.