Chosen Solution
So just the other day, my 2020 iMac screen went black, the power completely cut, and and no amount of holding the power button would get it to come back. I ended up having to unplug it from the wall, and then it would boot up again. This blackout happened a few more times that day, I tried shuffling around/removing RAM sticks and running the diagnostics, but nothing came back with issues or solved the problem. Eventually it stoped crashing, so I backed up some files and kept using it as normal. Last night it started blacking out again, and now it can’t even get through the boot sequence, and safe boot makes no difference. This has now made me believe that it’s a hardware failure, and not a software issue. The closest thing I could determine is maybe a faulty PSU, but I really wanted to know if there were more rigorous tests I could do before ordering and installing a new one. Does anyone have any other ideas on what could be the problem, and how I could diagnose with more certainty? Side note: I’m also remembering that I have had an occasional problem with this computer since I bought it (new) where the fans would suddenly spin up to maximum, sometimes even when it was sleeping, and none of the fan control applications I tried would ever actually register the RPMs. Sometimes they’d spin up for 20 seconds, and other times I had to shut down the computer and unplug it from the wall to make them stop. I don’t know enough about the hardware of this computer to make real educated guesses, but misbehaving fans and power management sound somewhat related to the SMC? Could the SMC itself be faulty? Thanks, Jak
SMC logic issues are rare, more often the sensor has detected a failure, and SMC goes into a latched state, so when you did disconnected the AC plug and held the power button you cleared the latch state and then the system works again until it hits the failure issue again. If you are still able to get it to restart I would run the onboard diagnostics to see if an error code pops, if it does post it here so we can have a starting point. If the system won’t start then you’ll need to carefully remove the display so you can check the diagnostic LED’s, review this for the details Diagnostic LEDs and Test Pads for iMac (Retina 5K, 27-inch, 2019) they are the same for the 2020 model as well. I’ll warn you you need to use great care and use the proper tools and techniques as its very easy to damage the display! iMac Intel 27" Retina 5K Display 2020 Display Assembly Replacement Here’s the needed tools: Pro Tech ToolkitiMac Intel 27" Cardboard Service WedgePlastic Cards I would get two sets.iMac Opening Wheel, if you get the replacement adhesive strips the tool is in the kit iMac Intel 27" (2012-2019) Adhesive Strips Which I strongly recommend getting two strip kits. Also you’ll want some painters tape to hold the display when needed.
