when I back car up, my parking assist shuts off on 2004 Lincoln Town Car

As soon as I put is reverse, it say park assist disarmed.
It's a 2004 Lincoln town car with 29,000 miles on it . Car was taken care of, car fax said one owner. please help.

Asked by for the 2004 Lincoln Town Car
check owner manual as this system can be turned off. if it is on, may have a bad sensor in rear bumper.
the system works correctly if you are within inches of an object, if you keep shifting to reverse, otherwise it says system disarmed the on and off switch overhead has no effect
this sounds like a sensor problem. will need to go to a dealer to be diagnosed.
1 more answer
I have the same problem on a 2004 signature , if you are really close on something backing up it works correctly, otherwise it shuts off .