GMC 2007 Acadia Owner's Manual page 202

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 the
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 less
than 40°F (4°C). 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 most 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 less than 40°F (4°C).
Do not drive the vehicle until all the windows
are clear.
AUX (Auxiliary): Press this button to turn the
rear heating and air conditioning on. See Rear Air
Conditioning and Heating System on page 211
or Rear Air Conditioning and Heating System and
Electronic Climate Controls on page 212.