2008 GMC Yukon Owner's Manual - Page 206

Defogging and Defrosting Fog on the inside of the 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. There are two modes to clear fog or frost from your windshield. Use the mode up and down arrows to select the defog or defrost modes. Rear Window Defogger The rear window defogger uses a warming grid to remove fog from the rear window. < (Rear Window Defogger): - (Defog): The defog mode is used to clear the windows of fog or moisture and warm the passengers. This mode directs air to the windshield, floor outlets, and side window vents. When this mode is selected, the system turns off recirculation and runs the air conditioning compressor unless the outside temperature is close to freezing. The recirculation mode cannot be selected while in the defog mode. Do not drive the vehicle until all the windows are clear. 0 (Defrost): Press this button to remove fog or frost from the windshield more quickly. This mode directs a portion of the air to the windshield and side window vents and some to the floor vents. In this mode, the system will automatically force outside air into the vehicle. The recirculation mode cannot be selected while in the defrost mode. The air conditioning compressor will run automatically in this setting, unless the outside temperature is close to freezing. Do not drive the vehicle until all the windows are clear. Press this button to turn on the rear window defogger. It will automatically turn off several minutes after it has been activated. The defogger can also be turned off by pressing the button again or by turning off the engine. Do not drive the vehicle until all the windows are clear. If your vehicle has heated outside rearview mirrors, the mirrors will heat to help clear fog or frost from the surface of the mirror when the rear window defog button is pressed. Notice: Do not use a razor blade or sharp object to clear the inside rear window. Do not adhere anything to the defogger grid lines in the rear glass. These actions may damage the rear defogger. Repairs would not be covered by your warranty. 3-32