Garage Cabinets, Garage Storage, and Garage Organization

I have a 2003 Ford Expedition.  I keep getting this message "check back up aid".....I have cleaned the bumper area in the back.  It still does not work.  I really like this feature and feel very uncomfortable without it working.  Has anyone else had this happen? If so, what do I need to do?   Thanks

I also have expedition 2003 with the same problem , i have taken my car to the ford agency , they have taken alredy two dayes to trobelshoot the prblem and have indicated that  i need to by two parts  250$ to fix the problem , but then they seam not so sure so they indicated that i need to by a third part 270$ total for the three parts 520$ , witch is to much i beleive to fix this problem .
conclusion :
I have reclined to buy these parts and I'm working on a research on this problem to realy fined out what is wrong and try to fix it with the least coast possible.