GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 215

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(Recirculation): Press to turn the recirculation
mode on. An indicator light comes on to show that the
recirculation is on.
This mode recirculates and helps to quickly cool the air
inside the vehicle. It can be used to help prevent outside
air and odors from entering the vehicle.
The recirculation mode cannot be used with floor, defog,
or defrost modes. If recirculation is selected with one of
those modes, the indicator light flashes three times and
then turns off. The air conditioning compressor also
comes on when this mode is activated. While in
recirculation mode the windows may fog when the
weather is cold and damp. To clear the fog, select either
the defog or defrost mode and increase the fan speed.
The recirculation mode can be turned off by pressing the
outside air button, or by turning off the ignition.
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(Outside Air): Press to turn the outside air mode on.
An indicator light on the button comes on to show that
outside is on. When selected, air from outside the vehicle
circulates throughout the vehicle. The outside air mode
can be used with all modes, but it cannot be used with the
recirculation mode.
Rear Window Defogger
The rear window defogger uses a warming grid to
remove fog from the rear window.
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(Rear Window Defogger): For vehicles with
this feature, press to turn the defogger on or off.
It automatically turns off several minutes after it has
been activated. The defogger can also be turned off by
turning the engine off. Do not drive the vehicle until all
the windows are clear.
Notice: Do not use a razor blade or sharp object to
clear the inside rear window. Do not adhere anything
to the defogger grid lines in the rear glass. These
actions may damage the rear defogger. Repairs
would not be covered by your warranty.
Heated Mirrors: For vehicles with heated outside
rearview mirrors, the mirrors heat to help clear fog or frost
from the surface of the mirror when the rear window
defog button is pressed. See Outside Power Mirrors on
page 2-58.
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