Hi folks,
I've checked this site out a long time and used the info here to help me understand and work on my Mercury Monterey. I figured I'd finally register and try to chip in some assistance if someone has a problem. This post I wanted to point out something to look for if your parking sensor system malfunctions (also known as the RSS or FSS).
You may want to look at the speaker itself rather than just the sensors. I bought a sensor off Ebay for $45 and tried replacing each of the 8 sensors one by one on my car after my system stopped working. It took hours to remove the bumpers and replace them one by one. The system still wouldn't work after doing all this, so I figured I'd spend the money and have the dealer run a diagnostic check.
The dealer charged $90 to hook it up and run the codes and it turns out it was the front speaker that makes the beeping noise above the interior fuse panel that was bad. Water had somehow dripped down the wire from below the cowl near the wipers and trailed down into the speaker terminals. It ruined the speaker and the whole system shut off. There was corrosion inside the speaker itself.
Now here's the best part. Get this, the ford stealership by me said I needed to replace a rear sensor also that was bad (part cost $220), which I didn't bother to do and it didn't need replacing after all. Total price quote to fix the parking sensor system...pause here and drumroll please...............$1400+!!!!!!!!!!!!!!!!!! Holy smokes!!! They said they had to take the dashboard off to get at the speaker.
Instead I orderd the speaker from Fordparts.com. It cost $22 for the part and all I had to do was take off 5 screws holding the trim panels in place below the steering column to get at it. Half hour max. I put in the key and bingo it works now. I didn't even need to replace the sensor that they said went bad.
Moral here is what an amazing rip-off they were trying to scam me for. So watch out folks, you are far better off buying a cheap set of service manuals off of Ebay like I did because anybody could do this repair. The dealership may be useful to help diagnose the problem but a lot of this stuff can be repaired by yourself.