GMC Yukon Denalid Owner's Manual page 220

The interior temperature sensors located in the
headliner above the driver side seat and in
the headliner above the second row seats
measure the temperature of the air inside your
vehicle.
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There is also an exterior temperature sensor
located behind the front grille. This sensor reads
the outside air temperature and helps maintain the
temperature inside the vehicle. Any cover on
the front of the vehicle could cause a false reading
in the displayed temperature.
The climate control system uses the information
from these sensors to maintain your comfort setting
by adjusting the outlet temperature, fan speed, and
the air delivery mode. The system may also supply
cooler air to the side of the vehicle facing the sun.
The recirculation mode will also be used as needed
to maintain cool outlet temperatures.