Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I think you should take apart each connector involved in the signal train (from the ST850 to the Furuno display device) one by one - brush each connector, both sides of the connection with a clean dry brush, then wipe the metal down with a clean dry rag cloth until the metal shines, then re-connect firmly.
Corrosion on connections, in a marine electronics context, can be very important to the non-function of devices - so such routine maintenance can do a lot for a non-working device, on occasion.
After that... yes, if devices at both ends aren't communicating, swapping out another element in the linkup between them, makes sense.
As for "the Y-connector" - I don't see your setup, I don't know about what's hooked up to what, so I really cannot advise on that detail, until I get the whole picture, or at least more of the whole picture.
Let me know how it goes. Thanks.