GMC 2005 T-Series Owner's Manual page 139

You can run the engine to keep warm, but be careful.
CAUTION:
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Snow can trap exhaust gases under your
vehicle. This can cause deadly CO (carbon
monoxide) gas to get inside. CO could
overcome you and kill you. You cannot see it or
smell it, so you might not know it is in your
vehicle. Clear away snow from around the base
of your vehicle if any is blocking your exhaust
pipe. And check around again from time to time
to be sure snow does not collect there.
Open a window just a little on the side of the
vehicle that is away from the wind. This will help
keep CO out.
Run your engine only as long as you must. This saves
fuel. When you run the engine, make it go a little
faster than just idle. That is, push the accelerator
slightly. This uses less fuel for the heat that you get and
it keeps the battery (or batteries) charged. You will
need a well-charged battery (or batteries) to restart the
vehicle, and possibly for signaling later on with your
headlamps. Let the heater run for a while.
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