Buick 2002 Park Avenue - Manual (Page 163)

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A chime will sound the first time an object is detected,
if the object is between 20 inches (0.5 m) and 5 feet
(1.5 m) away.
URPA can detect objects 3 inches (7.6 cm) and wider,
and at least 10 inches (2.5 cm) tall, but cannot detect
objects that are above trunk level. In order for the rear
sensors to recognize an object, it must be within
operating range.
If the URPA system is not functioning properly, the
display will flash red, indicating that there is a problem.
The light will also flash red when you are driving if a
trailer is attached to your vehicle, or if you have a
bicycle or an object hanging out of your trunk. The
light will continue to flash until the trailer or the object
is removed and your vehicle is driven forward at least
15 mph (25 km/h). It may also flash red if the ultrasonic
sensors are not kept clean. For cleaning instructions, see
“Cleaning Your Vehicle” in the Index.
So be sure to keep your rear bumper free of mud,
dirt, snow, ice and slush or the system may not work
properly. If after cleaning the rear bumper and then
driving at least 15 mph (25 km/h), the display continues
to flash red, see your dealer. It may also flash red if your
vehicle is moving in REVERSE (R) at a speed greater
than 3 mph (5 km/h). Other conditions that may affect
system performance include things like, the vibrations
from a jackhammer or the compression of airbrakes
on a very large truck.
As always, drivers should use care when backing up a
vehicle. Always look behind you, being sure to check
for other vehicles, obstructions and blind spots.