GMC 2006 Envoy Owner's Manual page 170

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A/C (Air Conditioning): Press this button to
manually turn the air conditioning system on or off.
When the system is on, the system will automatically
begin to cool and dehumidify the air inside of your
vehicle. The air conditioning symbol will appear on the
display when the air conditioning is on and will turn
off when the air conditioning is off.
If you select turn the air conditioning off while in front
defrost or defog mode, the air conditioning symbol
will turn off, however, the A/C compressor will remain
on to help de-humidify the air inside the vehicle. If one of
the other modes is selected the compressor will then
turn the A/C off until it is selected again or the
AUTO button is pressed.
You may notice a slight change in engine performance
when the air conditioning compressor shuts off and
turns on again. This is normal.
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Defogging and Defrosting
Fog 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. You can use either defog or front defrost
to clear fog or frost from your windshield.
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(Defog): Use this setting to clear the windows of
fog or moisture. This setting will deliver air to the
floor and windshield outlets.
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(Defrost): Press the front defrost button to clear
the windshield and side windows of frost or fog quickly.
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. Do not
drive the vehicle until all the windows are clear.