Pontiac 2005 Montana SV6 Owner's Manual page 160

If an object is detected at a REVERSE (R) speed of
less than 3 mph (5 km/h), one of the following will occur:
Description
amber light
amber/amber lights
amber/amber/red lights &
continuous chime
amber/amber/red lights
flashing & continuous
chime
A chime will sound the first time an object is detected
between 20 inches (0.5 m) and 5 feet (1.5 m) away.
URPA cannot detect objects that are above liftgate level.
In order for the rear sensors to recognize an object, it
must be within detection range behind the vehicle.
3-22
English Metric
5 ft 1.5 m
40 in 1.0 m
20 in 0.5 m
1 ft 0.3 m
When the System Does Not Seem to Work
Properly
The light may flash red when the vehicle is in
REVERSE (R) if the ultrasonic sensors are not kept
clean. So be sure to keep your rear bumper free of mud,
dirt, snow, ice and slush. Other conditions that may
affect system performance include things like the
vibrations from a jackhammer or the compression of air
brakes on a very large truck. If after cleaning the
rear bumper and then driving forward at least 15 mph
(25 km/h), the display continues to flash red, see
your dealer.
If a trailer was attached to your vehicle, or a bicycle or
an object was on the back of, or hanging out of your
liftgate during your last drive cycle, the light may
also flash red. The light will continue to flash whenever
in REVERSE (R) until your vehicle is driven forward
at least 15 mph (25 km/h) without any obstructions
behind the vehicle.