GMC 2007 Yukon XL Owner's Manual page 212

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. Turn the right knob clockwise to select
the defog or defrost mode.
-
(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 you select this mode, 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.
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(Defrost): The defrost mode is used 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 your 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.