Hummer 2003 H2 Owner's Manual page 120

CAUTION:
-
All wheel drive vehicles with the transfer case in
NEUTRAL will allow the vehicle to roll, even if
your shift lever is in PARK (P). So, be sure the
transfer case is in a drive gear
Always set your parking brake.
Follow the proper steps to be sure your vehicle won't
move. See "Shifting Into PARK (P)" in the Index.
If you're pulling a trailer, see "Towing a Trailer" in
the Index.
2-38
--
not in NEUTRAL.
Electronically Controlled Air
Suspension System (If Equipped)
The electronically controlled air suspension keeps the
rear of your vehicle level as you load and unload it.
The system includes a compressor, two height sensors
and two air springs supporting the rear axle.
The engine must be running for the system to
operate and level the vehicle after loading or unloading.
The system will suspend leveling if any of the doors
or the liftgate are open. Once the doors are closed,
system leveling will continue. The system can lower
the vehicle to the standard ride height with the engine
running and for up to 30 minutes after the ignition has
been turned off.
You may hear the compressor running when you load
your vehicle, and periodically as the system adjusts the
vehicle to the standard ride height.
Load leveling will not function normally with the
inflator hose attached to the inflator outlet. Remove
the inflator hose from the outlet when it is not in use.
See "Accessory Inflator" in the Index.