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So I just opened an iMac that doesn’t give any sign of life (black screen) The onboard diagnostic LED-1 is on as soon as I plug in the AC power cable, I can also hear the HDD turning on when the power cable is plugged in. I can turn the HDD on and off by pressing the power button for 3 seconds but everything else seems to be off (fans not running) also LED-2 stays off when I plug in display, so only LED-1 is on everything else is off. Do you think the power supply is not giving enough energy for every part? Or do you think its a logic board problem?
If LED-2 never comes on that implies the Power Supply has failed. I would ask the customer if the system was plugged into a good surge suppressor or UPS. As its likely a power line surge or lightning strike damaged the power supply and it could have also damaged the logic board as well. I would test the power output connector pads to see what it tells me. iMac Intel 27" (Late 2012-2020) Power Supply and here’s the guide you’ll need to follow iMac Intel 27" EMC 2639 Power Supply Replacement
