from the owners manual :

Automatic Central Locking
The central locking switch also operates the automatic
central locking feature. With the automatic central locking
feature activated, the doors and rear liftgate/decklid
are locked at vehicle speeds of approximately 9 mph (15
km/h) or more; however, the fuel filler door remains
unlocked.
To activate this feature, turn the key to the ON/RUN
position and hold the upper portion of the switch for a
minimum of five seconds. To deactivate, turn the key to
the ON/RUN position and hold the lower portion of the
switch for a minimum of five seconds.
NOTE: If the doors are unlocked with the central
locking switch after activating the automatic central
locking feature and neither door is opened, the doors
remain unlocked even at vehicle speeds of approximately
9 mph (15 km/h) or more.