GMC 2006 Sierra - Manual (Page 194)

Your vehicle is equipped with a light sensor located on
the top of the instrument panel in the defroster grille.
Be sure it is not covered, or the system will be on
whenever the ignition is on.
The system may also turn on your headlamps when
driving through a parking garage, heavy overcast
weather or a tunnel. 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 will only be affected when the light sensor sees
a change in lighting lasting longer than the delay.
If you start your vehicle in a dark garage, the automatic
headlamp system will come on immediately. Once you
leave the garage, it will take approximately one minute for
the automatic headlamp system to change to DRL if it is
bright enough outside. During that delay, your instrument
panel cluster may not be as bright as usual. Make sure
your instrument panel brightness control is in the full
bright position. See Instrument Panel Brightness on
page 3-18.
To idle your vehicle with the automatic headlamp
system off, turn the control to the off position.
You may be able to turn off your Automatic Headlamp
System. See Daytime Running Lamps (DRL) on
page 3-15 for more information.
Your headlamps will also stay on after you exit the
vehicle. You can program this feature using the Driver
Information Center (DIC). See DIC Vehicle Customization
on page 3-64.
As with any vehicle, you should turn on the regular
headlamp system when you need them.
3-16