GMC 2003 C4C042 Owner's Manual page 88

Theft
Vehicle theft is big business, especially in some cities,
and even commercial vehicles are no exception.
Although your vehicle has a number of theft
features, we know that nothing we put on it can make
it impossible to steal. However, there are ways you
can help.
Key in the Ignition
If you leave your vehicle with the keys inside, it's an
easy target for joy riders or professional thieves
- don't do it.
deterrent
Remember to remove your key and take it with you.
Always do this. When you take your key with you,
your ignition will be locked. Also remember to lock
the doors.
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