GMC Yukon 2007 Owner's Manual page 236

Sensors
The solar sensor, located in the defrost grille in
the middle of the instrument panel, monitors
the solar radiation. Do not cover the solar sensor
or the system will not work properly.
236
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.
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.