GMC 2007 Sierra 1500 Pickup Owner's Manual page 224

The system may also turn on the 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 detects 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 226.
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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 222 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 282.
As with any vehicle, you should turn on the
regular headlamp system when needed.