Cadillac 2002 Escalade Owner's Manual page 177

A chime will sound the first time an object is detected.
URPA can detect objects 3 inches (7.6 cm) and wider,
and at least 10 inches (25.4 cm) tall, but it cannot detect
objects that are above the center of the Cadillac crest
badge. In order for the rear sensors to recognize an
object, it must be within operating range.
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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 while driving if a trailer or
trailer hitch ball mount is attached to your vehicle or if a
bicycle or object is on the back of, or hanging out of
your vehicle. 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. 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
driving forward 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 air brakes 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.
For cleaning instructions, see "Cleaning Your Vehicle"
in the Index.