GMC Sierra Classic 2007 Owner's Manual page 240

In order to prevent false temperature readings at
startup, the displayed temperature will not
change until the following occurs:
•
Vehicle speed is above 10 mph (16 kmh) for
5 minutes.
•
Vehicle speed is above 32 mph (51 kmh) for
2 and a half minutes.
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
You can use either defog or front defrost to clear
fog or frost from your windshield. Use the
defog mode to clear the windows of fog or
moisture. Use the front defrost button to defrost
the front windshield.
240
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(Defog): Use this setting to clear the windows
of fog or moisture. Press the mode button to
select this setting. This setting will deliver air to
the floor and windshield outlets.
0
(Front Defrost): Press the front defrost button
to defrost the windshield. The system will
automatically control the fan speed if you select
defrost from AUTO mode. If the outside
temperature is 40°F (4°C) or warmer, your air
conditioning compressor will automatically run to
help dehumidify the air and dry the windshield.
Rear Window Defogger
If your vehicle has this feature, the rear window
defogger uses a warming grid to remove fog from
the rear window.
If your vehicle has a sliding rear window, you will
not have a rear window defogger.
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(Rear): Press this button to turn the rear
window defogger on or off. An indicator light in the
button will come on to let you know that the
rear window defogger is activated. This button will
also activate the heated mirrors, if the vehicle
has them.