GMC 2009 Sierra - Manual (Page 159)

This mode keeps outside air from entering the vehicle.
It can be used to reduce outside air and prevent
odors from entering the vehicle. Recirculation may also
help cool the air inside the vehicle more quickly once
the temperature inside the vehicle is less than the
outside temperature.
The recirculation mode cannot be used with floor, defog
or defrost modes. If recirculation is selected with one
of these modes, the indicator light will flash three times
and then turn off. 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.
#
(Air Conditioning): Press this button to turn the
air conditioning system on or off. When this button
is pressed, an indicator light on the button will come
on to let you know the air conditioning is activated.
The air conditioning can be selected in any mode as
long as the fan switch is on.
The air conditioning system removes moisture from the
air, so you may sometimes notice a small amount of
water dripping underneath your vehicle while idling
or after turning off the engine. This is normal.
Defogging and Defrosting
Fog or frost on the inside of 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. To avoid fogging windows on
rainy and humid days at temperatures above freezing,
run the air conditioning compressor. It is best to
avoid the use of the recirculation mode, except when
maximum A/C performance is needed or for short times
to avoid exterior odors.
Turn the right knob to select the defog or defrost mode.
-
(Defog): This mode clears the windows of fog or
moisture and warm the passengers. The air is directed to
the windshield, floor outlets, and side window vents.
0
(Defrost): This mode removes fog or frost from the
windshield more quickly. A portion of the air is directed to
the windshield and the side window vents, with some air
directed to the floor vents. The system will automatically
force outside air into the vehicle.
The recirculation mode cannot be selected in the defog
or defrost mode. When either mode is selected, the
system runs the air conditioning compressor, unless the
outside temperature is close to freezing. Do not drive
the vehicle until all the windows are clear.
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