Only 1 (inner one on driver's side) of the 4 sensors on my GT (2005) is working. Came real close to hitting a pole when reversing in a tight spot the other night. Fortunately I am not the trusting type and played it safe. With motor running and car in Reverse one can even cover 3 of the sensors without any beeping noises occurring. The inner RHS sensor beeps correctly (slow and then faster as the distance is reduced). How often do the actual sensors fail and how accessible are they without removing bump? How are they retained? Best place to get replacements?

Bumper has to come off...it's not hard.
They fail all the time, but cleaning often revives them.

I had cheap $40 eBay ones that worked without fail for 5 years until I got rear-ended. The replacement were also faultless. I would check the control module to see if the plugs have come lose or not before removing the bumper.