In addition to what I wrote in the original post, I forgot to mention that I made and used the DIY code reader to read and reset all my codes. None of them have reappeared, so I have no codes from the EDS, further adding to the mystery.

Please help! I'm close to having to make the reluctant decision to either replace the transducer, or go to the Merc mechanic and pay some ungodly fee to have them tell me what needs to be done.