I had a lot of six Naomi motherboards in for repair recently, mostly just replacing rams so nothing worth logging but one of them was interesting. It would boot but would not enter test and the dip settings were stuck on, I traced the dip switch traces to the other side of the board. To a Sega custom chip 315-6146, I decided to replace the ram connected to it first as that is was much easier to swap and thankfully it was just the ram, a common 62256 type.
While playing with Naomi stuff I found that the Net Dimm board that I have owned and used for a good 15+ years was slightly faulty. It wouldn't return a serial number, this isn't usually a problem apparently because I haven't noticed an issue before, not until I tried to load Ikaruga and got error 4 that is.
A google search brought up the exact issue, serial number not returned by Dimm board but not many people with the issue, I only found one forum post with someone else with the error. It doesn't look like they ever fixed it either.
The serial number is stored on a tiny EEPROM on the bottom of the Dimm board at IC6S
That got her going.