2008 GMC Sierra 1500 Pickup Owner's Manual - Page 159

This mode keeps outside air from entering the vehicle. It can be used to reduce outside air and prevent odors from entering the vehicle. Recirculation may also help cool the air inside the vehicle more quickly once the temperature inside the vehicle is less than the outside temperature. The recirculation mode cannot be used with floor, defog or defrost modes. If recirculation is selected with one of these modes, the indicator light will flash three times and then turn off. While in recirculation mode the windows may fog when the weather is cold and damp. To clear the fog, select either the defog or defrost mode and increase the fan speed. The recirculation mode can be turned off by pressing the outside air button, or by turning off the ignition. Defogging and Defrosting Fog or frost on the inside of windows is a result of high humidity (moisture) condensing on the cool window glass. This can be minimized if the climate control system is used properly. To avoid fogging windows on rainy and humid days at temperatures above freezing, run the air conditioning compressor. It is best to avoid the use of the recirculation mode, except when maximum A/C performance is needed or for short times to avoid exterior odors. Turn the right knob to select the defog or defrost mode. - (Defog): This mode clears the windows of fog or moisture and warm the passengers. The air is directed to the windshield, floor outlets, and side window vents. This mode removes fog or frost from the windshield more quickly. A portion of the air is directed to the windshield and the side window vents, with some air directed to the floor vents. The system will automatically force outside air into the vehicle. The recirculation mode cannot be selected in the defog or defrost mode. When either mode is selected, the system runs the air conditioning compressor, unless the outside temperature is close to freezing. Do not drive the vehicle until all the windows are clear. # (Air Conditioning): Press this button to turn the air conditioning system on or off. When this button is pressed, an indicator light on the button will come on to let you know the air conditioning is activated. The air conditioning can be selected in any mode as long as the fan switch is on. The air conditioning system removes moisture from the air, so you may sometimes notice a small amount of water dripping underneath your vehicle while idling or after turning off the engine. This is normal. 0 (Defrost): 3-23