I’ve been following this thread and having the same exact issue as the OP. I’ve got the 63qt Elements Fridge that I bought in 2018. Fridge quit working on the 12v side in 2021. I threw it in my garage and forgot about it till it was three months out of warranty in 2022. ARB would not warranty it. My bad, life was busy in 2020-2022! The tests in the service manual don’t actually pertain to the exact issue when the DC function is not working but the AC is and the control panel still lights up on AC but not DC. Ran through the series of recommended tests and determined that it was most likely the compressor control unit based on what I could ascertain from the service manual. Ordered and replaced but did not fix the problem. Then I ordered and replaced the main circuit board and the control panel circuit board. Still no dice.
I am getting 12v power into the circuit boards, Thermistor tested and functioning as it should. Not sure what else to do at this point but it sucks having a $1,400 boat anchor. Please do yourselves a big favor and never purchase an ARB fridge. I’m now looking for another fridge that isn’t an ARB.