Hitachi P50S601 Y-Sustain Issue

Are Hitachi Y-SUS boards fragile?

I have 2 p50s601's. 1 was working, 1 was not. I decided to swap out every board one by one from the working to the non working one to try and determine which board was bad. The end result is now 2 NON working tv's!!!! both having a 4 blink code.....not the common 3.

My SDR boards are fine (no burn marks and all SMD caps check out) , both power supplies have new caps. Seeing on the forum that the majority of issues pertaining to the 601 tv's are centered around the SDR's and Power supplies.......there's little documented about the Y-boards. What actually goes bad on the Y-SUS board?

I have tried 4 different Y-SUS boards between these 2 tv's (keep in mind that 1 of the 4 boards was actually working). What would cause the one working Y board to go bad when placed in another tv? If they go bad, what goes bad on them and can it be fixed without buying a replacement? Are there certain caps that go bad on it? How about the Mosfets?

I am curious to hear if anyone has explored fixing a Y-SUS board and if so, please tell me how you did it....because I have 4 of them that I would hate to throw out.

Thanks in advance.