2012 GMC Sierra Owner's Manual - Page 271

Climate Controls 8-5 0 (Defrost): This mode removes fog or frost from the windshield more quickly. Air is directed to the windshield and the side window vents, with some air directed to the floor vents. The system automatically forces outside air into the vehicle. Do not drive the vehicle until all the windows are clear. Dual Automatic Climate Control System The heating, cooling, and ventilation in the vehicle can be controlled with this system. The vehicle also has a flow-through ventilation system described later in this section. A. Driver and Passenger Temperature Controls B. Fan Control C. AUTO D. Defrost E. Recirculation F. Outside Air G. Air Delivery Mode Control H. Display I. J. L. On/Off Rear Window Defogger PASS (Passenger) K. Air Conditioning