GMC 2006 Sierra - Manual (Page 116)

Ignition Positions
With the key in the ignition switch, you can turn it to
four different positions.
Notice: Using a tool to force the key from the
ignition switch could cause damage or break the key.
Use the correct key and turn the key only with your
hand. Make sure the key is all the way in. If it is, turn
the steering wheel left and right while you turn the
key hard. If none of this works, then your vehicle
needs service.
A (Lock): This position locks your ignition when the key
is removed. The steering locks when equipped with a
manual transmission. It is a theft-deterrent feature. You
will only be able to remove your key when the ignition is
turned to LOCK.
B (Accessory): This position allows you to use things
like the radio, power windows and the windshield wipers
when the engine is off.
{ 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 ACCESSORY. Do not press the
key release button while the vehicle is moving.
C (Run): This is the position for driving.
D (Start): This position starts your engine.
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