GMC 2009 Sierra - Manual (Page 166)

Air Conditioning
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(Air Conditioning): Press this button to turn the
air conditioning (A/C) compressor on and off. When
air conditioning is selected, an indicator light comes on
to show that the air conditioning has been activated.
Pressing this button when the outside temperature
is too cool for air conditioning makes the air conditioning
indicator flash three times and then turn off to let you
know the air conditioning mode is not available.
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 underneath
the vehicle while idling or after turning off the engine.
This is normal.
Sensors
The solar sensor, located
in the defrost grille, in the
middle of the instrument
panel, monitors the
solar radiation. Do not
cover the solar sensor
or the system will not
work properly.
The interior temperature
sensor, located in the
headliner, measures the
temperature of the air
inside the vehicle.
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