GMC 2006 Sierra 1500 Pickup Owner's Manual page 204

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 driver's set temperature, delivery
mode and fan speed. Press the AUTO button again
within five seconds to display the passenger's set
temperature.
If the driver and passenger's temperature settings
are not the same, the opposite side temperature
setting will be displayed for an additional
five seconds. To make the passenger's
temperature the same as the driver's press and
hold the AUTO button for about four 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's 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. Turn the driver's or
passenger's side temperature knob 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 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.
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.