By-Pass Air Control Valve
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 2002 Mazda Tribute, 2002 Mazda Protege5, 2002 Mazda Protege, 2002 Mazda Millenia, and 2002 Mazda MX-5 Miata. However, it is still accessible from the selected car via links, so may be relevant.
The By-Pass Air Control (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.