[weblacky]

Yo Ian,
This might be a dumb question but, could the loopback test failure simply be a symptom that the ethernet controller is generally unable to function?

For example like having a corrupt MAC address or bad local power rail? I'm assuming you have a functional battery on the board and have done some kind of check on the whatever ethernet controller chip VCC/GRD lines?

By check I don't mean while inside a running machine, I generally mean through other means such as providing proper voltage externally on the controller traces with a bench PSU or attempting a diode or Ohm check between VCC and GND for the ethernet chip?

Or perhaps a chip-wide diode check from pins-> grd as a simple profile?

Best of luck,
-Josh