Outside Air Button
WARNING: This page is about a different car, the 1999 GMC Sierra and 1999 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
When the outside air button is pressed, air from outside of the vehicle draws into the evaporator and heater core module. The air heats or cools depending on what temperature is set. The air can discharge from the air conditioner, the heater and the defroster outlets.