GMC 2010 Sierra Owner's Manual page 237

The vehicle has a light sensor located on the top of the
instrument panel in the defroster grille that regulates
when the automatic headlamps turn on. Do not cover
the sensor or the headlamps will come on whenever the
ignition is on.
The system may also turn on the headlamps when
driving through a parking garage or heavy overcast
weather. This is normal.
There is a delay in the transition between the daytime
and nighttime operation of the Daytime Running Lamps
(DRL) and the automatic headlamp systems so that
driving under bridges or bright overhead street lights
does not affect the system. The DRL and automatic
headlamp system is only affected when the light sensor
detects a change in lighting lasting longer than the
delay.
If the vehicle is started in a dark garage, the automatic
headlamp system comes on immediately. Once the
vehicle leaves the garage, it takes approximately
one minute for the automatic headlamp system to
change to DRL if it is bright enough outside. During that
delay, the instrument panel cluster may not be as bright
as usual. Make sure the instrument panel brightness
control is in the full bright position. See Instrument
Panel Brightness on page 4 15.
To idle the vehicle with the automatic headlamp system
off, turn the control to the off position.
The headlamps will also stay on after you exit the
vehicle. This feature can be programmed using the
Driver Information Center (DIC), if equipped. See DIC
Vehicle Customization (With DIC Buttons) on
page 4 76.
If the vehicle is not equipped with DIC buttons, exit
lighting is automatic. When it is dark enough outside,
the exterior lamps remain on for 30 seconds after the
ignition is moved from ON/RUN to LOCK/OFF.
For vehicles without a radio, the instrument panel light
remains on for 30 seconds with the driver's door closed.
For vehicles with a radio, the instrument panel light
remains on for 10 minutes with the driver's door closed.
See Retained Accessory Power (RAP) on page 3 25.
The regular headlamp system can be turned on when
needed.
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