My 4,1 (flahed 5,1) just dead few hours ago. No idea what happened. It runs 24/7 as server, at the tea time period, it still good. Just before I go for dinner, l realise that the computer hanged, black screen, no respon to the caps lock key, fan spining at higher RPM but not crazy speed. I force shut it down. And then it won't boot again. Power button is good. White light on, fan spining at normal speed, but no start up sound and nothing display. SMC reset, RTC reset, nil help. Unable to perform PRAM reset due to no start up sound and no display. Swap back the stock GPU, nil help. No warning light related to CPU or RAM, tested with only one RAM installed, nil help. Remove all PCIe card, nil help. Remove all HDD SSD, nil help. Only bootable SSD, nil help. Only bootable HDD, nil help. Reseat everything nil help. Disconnect everything except power cable, nil help. No CPU tray, won't power up as expected. All diagnostics LED normal. So, it seems the biggest suspect now is the motherboard. It won't even start the self test sequence. Anyone has similar case before? Any more test I can do? The only test I didn't do is go back to the W3520.