It's usually one of the 4 sensors in the bumper either not working or giving strange readings. Sometimes this can be caused by dirt on them or moisture from pressure washing.
If it comes to it the sensors are relatively inexpensive and can be changed without removing the rear bumper if you are prepared to carefully butcher the old one.
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2011 XC90 D5 Executive
2003 C70 T5 GT
2012 Ford Ranger XL SC
1977 Triumph Spitfire 1500
1976 Massey Ferguson 135
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