GMC Sierra 2008 Owner's Manual page 202

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
remains at the maximum cooling setting. If you
choose the temperature setting of 90°F (32°C),
the system remains 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.
For more information on the solar sensor, see "Sensors"
later in this section.
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To avoid blowing cold air in cold weather, the system
delays turning the fan on until warm air is available.
The length of delay depends on the engine coolant
temperature. Pressing the fan switch overrides this
delay and changes the fan to a selected speed.
O (On/Off): Press this button to turn off the climate
control system. Outside air still enters the vehicle,
and is directed to the floor. This direction can be
changed by pressing the mode button. Recirculation
can be selected once you have selected vent or bi-level
mode. The temperature can also be adjusted using
either temperature button. If the air delivery mode
or temperature settings are adjusted with the system off,
the display illuminates briefly to show the settings and
then returns to off. Press the on/off button or the up
down arrows on the fan switch, the defrost button,
AUTO button, or the air conditioning button to turn
the system on when it is off.