Service Park Assist?

I have a 2007 Tahoe LTZ with the Park Assist option. Lately the Park Assist is inoperable and the message "Sevice Park Assist" appears on the message center. I actually took the rear bumper cover off and cleaned the sensors thinking that maybe dirt/dust was making them unable to "see". No such luck. I've searched this forum and can't find anyone else encountering this problem. Is my only option to take it to the dealer?