My parkins sensors only work when i am pulling a trailer. Otherwise the off light is always iluminated. why is this?

 

One of the sensors is bad,They set a code but you will need a higher end scan tool (Don't waste time @ AutoZone)to read the codes,A OEM Ford Tool Will do it so will Autoengunity with the advanced level Ford software,There are usually replacement sensors on eBay for a reasonable price,You may try this to diagnose key on engine off & parking brake set put truck in reverse & take a length of heater hose or garden hose & listen to each sensor if the sensor is working you should be able to hear a sound from it.

 

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.