GMC 2007 Envoy Denali Owner's Manual page 193

If you 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.
To avoid fogging the inside glass on rainy and
humid days at a temperature above freezing, press
the air conditioning button to run the A/C
compressor. Also it is best to avoid the use of the
recirculation mode except when maximum air
conditioning performance is needed or for short
times to avoid exterior odors.
You may notice a slight change in engine
performance when the air-conditioning compressor
shuts off and turns on again. This is normal.
Defogging and Defrosting
Fog on the inside of the windows is a result of
moisture condensing on the cool window glass.
This can be reduced if the climate control system is
used properly. You can use either defog or front
defrost to clear fog or frost from your windshield.
-
(Defog): Use this setting to clear the windows
of fog or moisture. This setting will deliver air
to the floor and windshield outlets.
0
(Front 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.
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