GMC 2000 Sierra - Manual (Page 82)

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There are several ways to lock and unlock your vehicle.
You can use the keyless entry system (if equipped).
You can also use your key.
Use only the key or the keyless entry transmitter if your
vehicle is equipped with the content theft
- deterrent
system and the system is on.
From the inside, to lock the
door, slide the lever
rearward. To unlock
the door, slide the
lever forward.
Power Door Locks (If Equipped)
If your vehicle has this
power door lock switch,
press the raised side of the
switch, marked “L” on
either front door to lock all
the doors at once.
Press the recessed side of the switch marked “U” to
unlock all the doors at once.