Pontiac G3 2009 Owner's Manual page 190

You can run the engine to keep warm, but be careful.
4-16
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 the 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.