GMC 1999 Sierra 1500 Pickup Owner's Manual page 100

Leaving Your Vehicle With the Engine Running (Automatic
Transmission Models Only)
I
CAUTION
A
It can be dangerous to leave your vehicle with the engine running.
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Your vehicle could move suddenly if the shift lever is not fully in P
(Park) with the parking brake firmly set.
If you have a four-wheel drive with a manual transfer case shift lever
and your transfer case is in N (Neutral), your vehicle will be free to
roll, even if your shift lever is in P (Park). So be sure the transfer
case is in a drive gear-not in N (Neutral).
And, if you leave the vehicle with the engine running, it could
overheat and even catch fire. You or others could be injured. Don't
leave your vehicle with the engine running unless you have to.
If you have to leave your vehicle with the engine running, be sure your
vehicle is in P (Park) and your parking brake is firmly set, before you leave
it.
If you have a four-wheel drive, be sure that the transfer case is in a drive
gear-not in N (Neutral).
After you've moved the shift lever into the P (Park) position, hold the regular
brake pedal down. Then, see if you can move the shift lever away from
P
(Park) without first pulling it toward you.
If you can, it means that the shift lever wasn't fully locked into P (Park).
Parking Your Vehicle (Manual Transmission Models Only)
Before you get out of your vehicle, put your manual transmission in R
(Reverse) and firmly apply the parking brake.
If you have four-wheel drive, be sure your transfer case is in a drive gear.
Your vehicle could roll if it isn't.
If
you are parking on a hill, or if your vehicle is equipped to tow a trailer,
see "Parking on Hills" or "Towing a Trailer" in the Index.
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