GMC Yukon Denalid Owner's Manual page 216

Automatic Operation
AUTO (Automatic): When automatic operation
is active the system will control the inside
temperature, the air delivery, and the fan speed.
Use the steps below to place the entire system in
automatic mode:
1. Press the AUTO button.
When AUTO is selected, the display will
change to show the current temperature(s)
and AUTO will be lit on the display. The current
delivery mode and fan speed will also be
displayed for approximately 5 seconds.
When AUTO is selected, the air conditioning
operation and air inlet will be automatically
controlled. The air conditioning compressor will
run when the outside temperature is over about
40°F (4°C). The air inlet will normally be set to
outside air. If it is hot outside, the air inlet may
automatically switch to recirculate inside air to
help quickly cool down your vehicle. The light
on the button will illuminate in recirculation.
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2. Set the driver's and passenger's temperature.
To find your comfort setting, start with a 74°F
(23°C) temperature setting and allow about
20 minutes for the system to regulate. Use the
driver's or passenger's temperature buttons to
adjust the temperature setting as necessary.
If you choose the temperature setting of 60°F
(15°C), the system will remain at the maximum
cooling setting. If you choose the temperature
setting of 90°F (32°C), the system will remain at
the maximum heat setting.Choosing either
maximum setting will not cause the vehicle to
heat or cool any faster.
Be careful not to cover the solar sensor located
on the top of the instrument panel near the
windshield. This sensor regulates air temperature
based on sun load and also turns on your
headlamps. For more information on the solar
sensor, see "Sensors" later in this section.
To avoid blowing cold air in cold weather, the
system will delay turning on the fan until warm air
is available. The length of delay depends on
the engine coolant temperature. Pressing the fan
switch will override this delay and change the
fan to a selected speed.