GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 214

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(Defrost): This mode removes fog or frost from the
windshield more quickly. Air is directed to the windshield
and side window vents, with some directed to the floor
vents. In this mode, the system automatically forces
outside air into the vehicle and runs the air conditioning
compressor unless the outside temperature is close to
freezing. The recirculation mode cannot be selected while
in the defrost mode.
Do not drive the vehicle until all the windows are clear.
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(Air Conditioning): Press to turn the air conditioning
(A/C) compressor on and off. An indicator light comes
on to show that the air conditioning is on.
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If this button is pressed when the air conditioning
compressor is unavailable due to outside conditions, the
indicator flashes three times and then turns off. If the air
conditioning is on and the outside temperature drops
below a temperature which is too cool for air conditioning
to be effective, the air conditioning light turns off to show
that the air conditioning mode has been canceled.
On hot days, open the windows long enough to let hot
inside air escape. This helps to reduce the time it takes
for the vehicle to cool down. It also helps the system to
operate more efficiently.
The air conditioning system removes moisture from
the air, so a small amount of water might drip under
the vehicle while idling or after turning off the engine.
This is normal.