In my case, the motherboard was badly seated, 3 mm is enough to make the ports fail because the connectors doesn’t go all the way in and it’s very hard to see this problem just by using your eyes, take it apart and take a look at the ports assembly on your MB, you’ll notice a small bump/ball/thing in the middle, this bump has to fit into the iMac ‘case’, you can feel it when seating the MB on the case if you do it right