My backup sensors are not working.

I have gotten under the bumper and wriggled the wiring with no improvement.

How do I troubleshoot this? My initial thought is to unclip each sensor from the harness and wash the contacts with cleaning fluid. If corrosion has caused a problem maybe this will correct that?

Beyond that, how would I test for a bad sensor? Any idea of the cost of a replacement sensor, if it turns out that one is bad?

Thank you.