AIR Shut Off Valve
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Pontiac Grand Prix, 2003 Pontiac Bonneville, 2003 Chevrolet Monte Carlo, 2003 Chevrolet Impala, and 2003 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
The AIR shut off valve is vacuum operated. When the secondary air injection system is enabled, vacuum is applied to the valve. The vacuum opens the valve, and allows air from the AIR Pump to flow to the check valves.