Chevrolet 2002 Astro Owner's Manual page 147

Service AWD Light (If Equipped)
The SERVICE AWD light comes on to indicate that
there may be a problem with the drive system and
service is required. Malfunctions can be indicated by the
system before any problem is apparent, which may
prevent serious damage to the vehicle. This system is
also designed to assist your service technician in
correctly diagnosing a malfunction.
2-74
This light should come on
briefly when you turn on the
ignition, as a check to show
you it is working.
Tow/Haul Light
For more information, see "Tow/Haul Mode" in
the Index.
This light should come on
when the tow/haul mode has
been selected.