Chosen Solution
I’ve been having an odd issue with my iMac. It starting having this behavior where it will randomly shut down. Sometimes it’s an hour into using it, other times it’s when it’s booting up. Sometimes, when it chooses to shut down, it chimes and tries to restart several times. Chime - Chime - Chime. Sometimes it fully boots, other times it turns off and requires an SMC reset. Everything pointed to a bad PSU, so I ordered one from iFixIt. I pulled a LiteOn PSU, and installed the Delta brand PSU from iFixIt. The Delta replacement part was used. Pretty easy swap, however my issue persists. I thought maybe the PSU I got from iFixIt was bad. Contacted them, and they sent me a replacement. This time it was a LiteOn PSU. I swapped it out, and still have the issue. Last night, it played a looping movie all night long (never shut off). As soon as I got up this morning, starting opening apps and such, it shut off. My son plays Fortnite on this iMac. Sometimes he can play it for hours before it shuts down. Sometimes it lasts 10 minutes. Sometimes it won’t even get past the Apple logo on startup. Things I have tried: Using the computer in a different room (different circuit/breaker).Swapped the PSU twiceTried a new power cordInstalled new RAMDid a clean install of CatalinaChecked temps during heavy usage (all good)Viewed LED lights on logic board (all good) So what’s the deal? I probably have a bad logic board, right? Random bits of details: I discovered that the surge protector the iMac was plugged into was not grounded. This was before I replaced the PSU. I purchased a new surge protector, then replaced the PSU when that did not alleviate the issue. I have heard that these iMacs are picky about being grounded.I have a lot of devices on the same circuit/breaker. I have unplugged everything else (except the iMac) and the issue still persists.I have a new iMac in my office that works fine, so I don’t think I’m having a surge issue issue, or some other issue with the quality of electricity coming in.I’ve done lots of testing, and I guess I can chock most of it up to “chance.” For example, my son was playing XBOX (same room). When he turned it off, the iMac restarted (it was asleep at the time). When I tried to replicate, it did not happen. Idling:
During Video Encoding:
Idling after Encoding:
I think you need to dig deeper into your power services. Check your outlets with a cheap outlet checker like this one
Here’s a good vid on why A MUST HAVE TOOL - The Electrical Outlet Tester You also need to check your breaker panel ground connection it needs to hook up to a metal pipe exiting your home or tied to one or more grounding rods. Make sure the connections are not corroded up and if the rods are covered in green oxide they will need to be replaced. You should also ask your power provider for a power meter which monitors your power feed from them. All it does is plug into an outlet for a few days or better a good week. This should be a free service. Instead of using a surge suppressor get a good UPS like a APC SmartUPS unit as it fills in voids and holds sync of AC frequency if your power supplier looses it during power feed change-outs from the different suppliers.
Hi Andrew, I’m having the same problem with my 27” iMac I bought in 2016. I can’t update the OS because of the shut downs. I haven’t performed the extensive troubleshooting you have done, but it’s not my electrical system because it did the same thing at another house as well. I’ll be sorely disappointed if my expensive machine simply stops working after 6 years, and I won’t make that mistake again. Big Sur 11.0.1 (20B29)iMac (Retina 5K, 27-inch, Late 2015)3.2 GHz Quad-Core Intel Core i58 GB 1867 MHz DDR3AMD Radeon R9 M390 2 GB I live in a metropolitan area and don’t think it’s an environmental cause for the shut downs. Any solution to your problem? Meg
