Cadillac 2000 DeVille 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 trunk level. 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 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 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.