GMC 2001 Sierra 1500 Pickup Owner's Manual page 102

LOCK (B): This position locks your ignition and
-
transmission. It's a theft deterrent feature. You will only
be able to remove your key when the ignition is turned
to LOCK.
CAUTION:
On manual transmission vehicles, turning the key
to LOCK will lock the steering column and result
in a loss of ability to steer the vehicle. This could
cause a collision. If you need to turn the engine
off while the vehicle is moving, turn the key only
to OFF. Don't press the key release button while
the vehicle is moving.
NOTICE:
If your key seems stuck in LOCK and you can't
turn it, be sure you are using the correct key;
if so, is it all the way in? Turn the key only with
your hand. Using a tool to force it could break
the key or the ignition switch. If none of this
works, then your vehicle needs service.
OFF (C): This position lets you turn off the engine.
Use OFF if you must have your vehicle in motion while
the engine is off (for example, if your vehicle is
being pushed).
RUN (D): This is the position for driving.
START (E): This position starts your engine.
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