GMC 2009 Sierra - Manual (Page 178)

This light should come on briefly when you turn the
ignition key to RUN. If it does not come on then,
have it fixed so it will be ready to warn you if there is
a problem.
{ 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.
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 4-41.
Antilock Brake System
Warning Light
For vehicles with the
Antilock Brake System
(ABS), this light will come
on briefly when you
start the engine.
That is normal. If the light does not come on then, have it
fixed so it will be ready to warn you if there is a problem.
If the ABS light stays on, turn the ignition off, if the light
comes on when you are driving, stop as soon as it
is safely possible and turn the ignition off. Then start the
engine again to reset the system. If the ABS light still
stays on, or comes on again while you are driving, your
vehicle needs service. If the regular brake system
warning light is not on, you still have brakes, but you do
not have antilock brakes. If the regular brake system
warning light is also on, you do not have antilock brakes
and there is a problem with your regular brakes.
See Brake System Warning Light on page 3-41
For vehicles with a Driver Information Center (DIC),
see DIC Warnings and Messages on page 3-59 for all
brake related DIC messages.
3-42