Hummer 2003 H2 Owner's Manual page 197

The air delivery modes and temperature selections may
still be used while the system is off. This is possible
since outside air will always flow through the system as
the vehicle is moving forward unless the system is set to
recirculation mode. See "Recirculation" later in this
section.
(Recirculation): Press this button to limit the
amount of outside air entering your vehicle. The light on
the recirculation button will glow. This is helpful when
you are trying to limit odors entering your vehicle and
for maximum air conditioning performance in hot
weather. Press this button again to allow outside air to
enter the vehicle. The light on the recirculation button
will go off.
Pressing this button cancels the auto recirculation feature.
To resume the auto recirculation function, press the
AUTO button. Each time the vehicle is started, the
system will revert to the auto recirculation function.
If you select recirculation while in defrost, defog or
floor, the light on the button will flash and go out to let
you know this is not allowed. This is to prevent fogging.
When the weather is cool or damp, operating the system
in recirculation for extended periods of time may cause
fogging of the vehicle's windows. To clear the fog,
select either defog or front defrost. Be sure A/C off is
not selected. You will want to allow the air conditioning
to run automatically to help dehumidify the air.
(Air Conditioning): Press this button to turn the
air conditioning compressor on and off.
(Air Conditioning Off): When you turn the air
conditioning off, this symbol will appear on the display.
When air conditioning is selected or in AUTO mode,
the system will run the air conditioning automatically to
cool and dehumidify the air entering the vehicle. If you
select A/C off while in front defrost or defog, the A/C
off symbol will flash to let you know this is not allowed.
This is to prevent fogging.
(Front Defrost): Press the front defrost button
to defrost the windshield. 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.
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