GMC Sierra Classic 2007 Owner's Manual page 170

Leaving Your Vehicle With the Engine
Running (Automatic Transmission)
CAUTION:
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It can be dangerous to leave your vehicle
with the engine running. Your vehicle
could move suddenly if the shift lever is
not fully in PARK (P) with the parking
brake firmly set.
If you have four-wheel drive and your
transfer case is in NEUTRAL, your vehicle
will be free to roll, even if your shift lever
is in PARK (P). So be sure the transfer
case is in a drive gear — not in NEUTRAL.
And, if you leave the vehicle with the
engine running, it could overheat and
even catch fire. You or others could be
injured. Do not leave your vehicle with the
engine running unless you have to.
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If you have to leave your vehicle with the engine
running, be sure your vehicle is in PARK (P)
and the parking brake is firmly set before
you leave it. After you move the shift lever into
PARK (P), hold the regular brake pedal down.
Then, see if you can move the shift lever away
from PARK (P) without first pulling it toward you. If
you can, it means that the shift lever was not
fully locked into PARK (P).
Torque Lock (Automatic Transmission)
If you are parking on a hill and you do not shift
your transmission into PARK (P) properly,
the weight of the vehicle may put too much force
on the parking pawl in the transmission. You
may find it difficult to pull the shift lever out
of PARK (P). This is called "torque lock." To
prevent torque lock, set the parking brake and then
shift into PARK (P) properly before you leave
the driver's seat. To find out how, see "Shifting Into
PARK (P)" listed earlier in this section.
When you are ready to drive, move the shift
lever out of PARK (P) before you release the
parking brake.