2002 GMC Yukon Owner's Manual - Page 187

(Defrost): This setting directs most of the air through the windshield defroster outlets with air flow also delivered from the side window and floor outlets. When in defrost mode, the air conditioning compressor may run to assist in dehumidifying the air and limit window fogging. Mode Buttons (Outside Air): Press this button to force the system to use outside air. Press is again to close off the outside air. Pressing the outside air button will cancel the recirculation button. (Recirculation): Press this button to limit the amount of outside air entering your vehicle. This is helpful when you are trying to limit odors entering your vehicle or attempting to quickly cool down the interior. Recirculation may be selected with the fan control in OFF to limit odors, outside air and dust from entering your vehicle. Pressing the recirculation button will cancel the outside air button. A/C (Air Conditioning): Press this button to turn the air conditioning on and off. The system will cool and dehumidify the air inside the vehicle when the A/C light is on. When in AUTO, the A/C is automatically engaged when necessary. Air conditioning will be automatically disabled when the outside air temperature drops below a level at which air conditioning is ineffective. Mode Knob -- Automatic Operation When the system is set for automatic operation, sensors will control the air delivery mode. Air will come primarily from the floor or instrument panel outlets, with some air directed to the windshield to prevent fogging. When the system is in AUTO mode, the air conditioning light will turn on, and the recirculation and outside air lights turn off. With the system in full auto control (both the fan and mode knobs in AUTO), you still have the ability to override any function. However, continually overriding the outside air or the A/C compressor will limit the ability of the system to cool the vehicle quickly. 3-7