GMC 2007 Sierra - Manual (Page 254)

This condition is normal since the charging system
is not able to provide full power at engine idle.
As engine speeds are increased, this condition
should correct itself as higher engine speeds allow
the charging system to create maximum power.
You can only drive for a short time with the reading
in either warning zone. If you must drive, turn off
all unnecessary accessories.
Readings in either warning zone indicate a
possible problem in the electrical system. Have
the vehicle serviced as soon as possible.
Up-Shift Light
(Manual Transmission)
Your vehicle may have
an up-shift light.
It will come on when you should shift to the next
highest gear. It will help the vehicle get the
best fuel economy.
Brake System Warning Light
With the ignition on, the brake system warning
light will flash when you set the parking brake. The
light will flash if the parking brake does not
release fully. If you try to drive with the parking
brake engaged, a chime will sound when the
vehicle speed is greater than 3 mph (5 km/h).
Your vehicle’s hydraulic brake system is divided
into two parts. If one part is not working, the other
part can still work and stop you. For good
braking, though, you need both parts working well.
If the warning light comes on and a chime
sounds there could be a brake problem. Have
your brake system inspected right away.
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