Cadillac 2002 DeVille Owner's Manual page 202

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 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 while driving if a trailer is
attached to your vehicle, or a bicycle or object is on
the back of, or 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. So be sure to keep your rear bumper free of
mud, dirt, snow, ice and slush or materials such as paint
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. For cleaning instructions, see "Cleaning, Outside
of Your Vehicle" in the Index.
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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.