Ford 2008 F-150 Owner's Manual page 279

Driving
The RSS automatically turns on
when the gearshift lever is placed in
R (Reverse) and the ignition is ON.
An RSS control allows the driver to
turn the RSS on and off. To turn the
RSS off, the ignition must be ON,
and the gear selector in R
(Reverse). An indicator light on the
control will illuminate when the system is turned off. If the indicator
light illuminates when the RSS is not turned off, it may indicate a failure
in the RSS. The RSS will remain off until either the RSS control is
pushed again or the ignition switch is recycled.
Keep the RSS sensors (located on the rear bumper/fascia) free
from snow, ice and large accumulations of dirt (do not clean the
sensors with sharp objects). If the sensors are covered, it will
affect the accuracy of the RSS.
If your vehicle sustains damage to the rear bumper/fascia, leaving
it misaligned or bent, the sensing zone may be altered causing
inaccurate measurement of obstacles or false alarms.
279
2008 F-150
Owners Guide (post-2002-fmt)
USA