GMC 2002 Yukon XL Owner's Manual page 184

(Blend): Airflow is delivered through the floor
outlets, center floor console outlets (if equipped), the
windshield defroster outlets and the side window
defroster outlets. The air conditioning compressor may
run to dehumidify the air to prevent window fogging.
(Defrost): This setting directs most of the
air through the windshield defroster outlets, side
window defroster outlets and some air through the
floor and center floor console outlets (if equipped).
If air from the center floor console is not desired, or
more defroster performance is desired, turn the vents
off at the console. When in defrost mode, the air
conditioning compressor may run to dehumidify
the air and limit window fogging.
Mode Buttons
(Outside Air): Press this button to direct the
system to use outside air.
Pressing the outside air button will cancel the
recirculation button.
3-4
(Recirculation): Press this button to limit the
amount of outside air entering the vehicle. This is
helpful when you are trying to limit odors entering the
vehicle or attempting to quickly cool down the interior.
Pressing recirculation will recirculate air continuously.
Recirculation is used when the fan is OFF to limit odors,
outside air and dust from entering your vehicle. All
button lights will go off. The last setting selected will
activate when the blower is turned back on.
Pressing the recirculation button will cancel the outside
air button. To return to normal operation and to prevent
stale air, be sure to return to outside air.
Between floor and blend mode, outside air is forced to
reduce window fogging. If the recirculation button is
pressed, the LED indicator will flash three times
indicating that forced outside air is activated and
recirculation is not available.