GMC C series 2002 Owner's Manual page 48

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The square head key is for
the ignition switch only.
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The oval head key is for
the doors.
If you have the optional one
will operate all locks, including the ignition switch.
When a new vehicle is delivered, the dealer removes the
plugs from the keys and gives them to the first owner.
Each plug has a code on it that tells your dealer or a
qualified locksmith how to make extra keys. Keep the
plugs in a safe place. If you lose your keys, you'll be
able to have new ones made easily using this code.
NOTICE:
Your vehicle has a number of new features that
can help prevent theft. But you can have a lot
of trouble getting into your vehicle if you ever
lock your keys inside. You may even have to
damage your vehicle to get in. So be sure you
have extra keys.
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key system, the oval key
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