Chosen Solution
When dissembling the logic board for a RAM upgrade I accidentally broke a small piece of the logic board. iMac still works and seems not to fire up no issues in the diagnostics test. What is it that I broke? Anyone knows? It the small green circle in the image:
What you snagged was an Inductor which is part of the Ethernet sockets logic. Here’s a tighter view of the parts, as you can see there is four sets of these Inductors with their larger mating Transformer just above. So your system will work but Ethernet connections won’t work
