GMC 2009 Sierra - Manual (Page 167)

There is also an exterior temperature sensor located
behind the front grille. This sensor reads the outside air
temperature and helps maintain the temperature
inside the vehicle. Any cover on the front of the
vehicle could cause a false reading in the displayed
temperature on the rearview mirror.
The climate control system uses the information from
these sensors to maintain your comfort setting by
adjusting the outlet temperature, fan speed, and the
air delivery mode. The system may also supply
cooler air to the side of the vehicle facing the sun.
The recirculation mode will also be used as needed
to maintain cool outlet temperatures.
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.
Use
H
and
G
to select the defog or defrost modes.
-
(Defog): Press this button 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 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.
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