Pontiac Montana SV6 2007 Owner's Manual page 387

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, 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.
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 charged.
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