Buick 2002 Rendezvous - Manual (Page 218)

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How the System Works
If the rear park aid disable switch is not on, the rear
parking assist will turn on automatically when the shift
lever is moved into REVERSE (R) and the vehicle
speed is less then 3 mph (5 km/h). When the system
turns on, the three lights on the display will illuminate
for 1.5 seconds to let you know that the system is
working. The red light will flash to indicate that the
system is disabled if your vehicle is moving in
REVERSE (R) at a speed greater then 3 mph (5 km/h).
When shifting into REVERSE (R) with the vehicle
speed less than 3 mph (5 km/h) and an object is
detected, one of the following will occur:
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.
RPA 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 liftgate level. In order for the rear
sensors to recognize an object, it must be within
operating range.