Ford Mustang Owner's Manual page 67

Ignition
Understanding the Positions of the Ignition
The positions of the key in the ignition
ACC allows some of your vehicle's electrical
accessories such as the radio and the windshield
wipers to operate while the engine is not
running. You must push the key release button
to turn to the ACC position if your vehicle has a
manual transmission.
LOCK locks the steering wheel. It also locks the
automatic transmission gearshift lever.
LOCK is the only position that allows you to
remove the key. The LOCK feature helps to
protect your vehicle from theft.
OFF allows you to shut off the engine and all
accessories without locking the steering wheel or
the automatic transmission gearshift lever.
ON allows you to test your vehicle's warning
lights (except the brake system warning light) to
make sure they work before you start the
engine. The key returns to the ON position once
the engine is started and remains in this position
while the engine runs.
START cranks the engine. Release the key once
the engine starts so that you do not damage the
starter. The key should return to ON when you
release it. The START position also allows you
to test the brake warning light.
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