Hummer 2005 H2 Owner's Manual page 173

United States
This light should come on briefly when you turn the
ignition key to RUN. If it doesn't come on then, have it
fixed so it will be ready to warn you if there's a
problem.
If the light comes on while you are driving, pull off the
road and stop carefully. You may notice that the pedal is
harder to push or may go closer to the floor. It may
take longer to stop. If the light is still on, have the vehicle
towed for service. See Towing Your Vehicle on
page 4-64.
Canada
CAUTION:
{
Your brake system may not be working
properly if the brake system warning light is
on. Driving with the brake system warning light
on can lead to an accident. If the light is still
on after you have pulled off the road and
stopped carefully, have the vehicle towed for
service.
3-33