Chevrolet 2007 Silverado 1500 Pickup Owner's Manual page 399

You can run the engine to keep warm, but be
careful.
CAUTION:
{
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, especially any that 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.
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