Unexpected mains power shutdown

Unexpected mains power shutdown

My late 2015 iMac 27 5K developed a fault earlier this year: it started shutting down unexpectedly, initially once or twice a day up to every two to three minutes. I checked the power cord, used different power outlets in different parts of the house, booted in safe mode, checked the syslog file for any error messages which could explain this, no explanation found. Important: after each unexpected shutdown I had to reset the SMC to get the machine to react to the power-on button again.

Asked by: Guest | Views: 287

Total answers/comments: 2

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mcgyver89 [Entry]

"Dan

@danj

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Self employed contract worker

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mcgyver89 [Entry]

"Hi, I don't know if I have the exact same problem but it looks like this. Every time I turn on my iMac, it shuts down during the boot process and restarts. Sometimes up to 10 times in a row. When the login screen is reached and I log in, the iMac also always shuts down within a few seconds to a few minutes. However, if I use this iMac in target display mode (cmd-F2) with another iMac, it no longer shuts down and I can use the iMac all day. If I log in to the iMac and then do nothing with it (no mouse movement and no keyboard), the iMac also stays on. Does anyone know what kind of defect is causing this? And why does the iMac never shut down (reboots randomly) when i use it in target display mode??

FYI the iMac on the left is a newer one with no issues. I usually (after a lot of spontaneous reboots) use the right iMac in target display mode as a second monitor for the left iMac. At 5.25min i perform a PRAM reset twice (without positive result). All reboots in this video recording occur spontaneously.

I also already tested with other RAM modules with no effect. I have this same problem at different places (at home and at work) with properly grounded outlets.

I recorded my issue. You can see it on https://www.youtube.com/watch?v=eF6uiHwC..."