Hummer 2003 H2 Owner's Manual page 111

(4LO Lock): This setting sends maximum power
to all four wheels. You might choose this if you are
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driving off road in deep sand, deep mud, and climbing
or descending steep hills.
When in this mode, you can also choose to lock the rear
axle for additional traction. See "Locking Rear Axle"
later in this section.
CAUTION:
Shifting the transfer case to NEUTRAL can
cause your vehicle to roll even if the transmission
is in PARK (P). You or someone else could be
seriously injured. Be sure to set the parking
brake before placing the transfer case in
NEUTRAL. See "Parking Brake" in the Index.
NEUTRAL: Shift the transfer case to NEUTRAL only
when towing your vehicle. See "Towing" in the Index
for more information.
Indicator lights in the buttons will show you which
position the transfer case is in. The indicator lights will
come on briefly when you turn on the ignition and one
will stay on. If the lights do not come on, you should
take your vehicle to your dealer for service. An indicator
light will flash while shifting the transfer case. It will
remain illuminated when the shift is complete. If for
some reason the transfer case cannot make a requested
shift, it will return to the last chosen setting.
If the SERVICE 4WD message in the DIC stays on,
you should take your vehicle to your dealer for service.
See "Service 4WD" or "Driver Information Center" in
the Index for further information.
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