GMC 2008 Savana Van Owner's Manual page 216

You can run the engine to keep warm, but be careful.
4-18
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/or the fuel
operated heater exhaust system, if equipped.
And check around again from time to time to
be sure snow does not collect there. If your
vehicle has a diesel engine and a fuel operated
heater, see "Fuel Operated Heater (FOH)" in
the diesel engine supplement.
Open a window just a little on the side of the
vehicle that is away from the wind. This will
help keep CO out.