Model A1419 / EMC 2806 / Late 2014 or Mid 2015. 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (ID iMac15,1); EMC 2834 late 2015 / 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (iMac17,1) All with Retina 5K displays

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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.

Took it to the Apple store for a repair. As the machine displayed these unexpected shutdowns while at the store they replaced the logic board and the power supply. Machine worked fine for a few weeks and then started shutting down unexpectedly again. Back to the Apple store again where this time the machine apparently ran fine for one week without any problem.

Back home within two hours started shutting down again, now every two minutes, so unusable. Does anybody have any idea what could make the SMC shut down the machine? Is it a (false) high temperature warning?

Which other part might need replacing?

Could there be another problem causing damage to the logic board and/or power supply?

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Did you bring your system to a real Apple Store or an Apple authorized service center?

You'll likely need to verify if the logic board and power supply where truly replaced. From the sounds of it I suspect your power supply and/or the logic board still has a problem.

You might want to also check your home's AC power. Things that can mess you up is bad power coming into your home from your utility as well as poor building grounding. You'll need to get an electrician to check your house wiring and see if you can get a power meter from your local utility.

You also want a good power strip for surge suppression. If you can swing it get a UPS.

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Agree with Dan. Before I got my 1st iMac in Dec. I got learning curve advice from friends who are computer engineers.

  • They said iMacs are particularly sensitive to power spikes and wave form and to get a UPS (Uninterrupted Power Supply) before ever turning the iMac on. LoL! I thought UPS was a delivery service.
  • Went for a CyberPower Intelligent LCD model (825VA) with "simulated" sine wave only because this was the model they used on their iMac but they use other brands as well.
  • Was told I didn't need "pure" sine wave which are considerably more expensive.
  • These units are roughly the same price across all brands. The price changes with volts/watts/power you need or if you go for pure sine wave over simulate.

Cheers xoxo :)

Update (07/15/2017)

Since giving my comment almost one year ago. My 27" iMac I bought in Dec 2016 specs:

  • 4 GHz Intel Core i7
  • 32 GB 1867 MHz DDR3
  • AMD Radeon R9 M395X 4096 MB

Began doing a total freeze (can't exit or close apps using keyboard) Had to use the power button to restart. I would wait for 30 - 60 seconds and it restarts. I figured "whatever" it was just times-out.

Any ideas? It happened mostly when I was touring around using Google maps in chrome. It was in full screen in 3D. With 32 GB RAM I wouldn't think I was overloading it.

Looked in Apple forums and the most logical thing was it could be something from a 3rd party app causing that. Its a common issue with the late 2015 iMacs particularly (from what I've read) with the upgraded processor and Graphics.

Any ideas. I'm under AppleCare for another year and a half. But, they're just going to say delete all 3rd party apps.

Cheers etc


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Hello Kemble, thanks for your comment. Fully with you re UPS. Run all my home stuff off two APC ES 700 UPSs. If the problem is caused by spikes, they either manage to get through the UPS (unlikely) or there are significant EM pulses in my house. I will run a data logger for several weeks to monitor mains and environment.


Hans-Georg - Did you get an electrician to check your house wiring (ground)? You could have a bad device plugged in which has a bad power supply messing up the ground (hot). Let us know if they find anything and your loggers output.


Thanks Dan, Apparently my comment to your original answer went awry. As I indicated in my other comment, I fully agree with your approach and suggestions. If the logger shows any spikes I will certainly get an electrician to check my mains supply in my house. I know from previous experience in Luxembourg that one device with a bad earth connection can cause a lot of problems. The only doubt I have about the spike theory is that the iMac showed the same type of unexpected shutdowns while I demo-d it to the chap at the Apple store. Will keep you informed about progress once I am back in the UK.


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I am having the same issue on my mid-2015. I have a mini Mac on the same power strip and it doesn't have a power shutdown issue.

Of course my warranty is over. What parts would cause this?

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Sean - Make your sure your power is clean and your house ground is good. I would first move your Mac mini to a different circuit so you can be sure you'er not over loading your line. Dedicate the line for your system and use a good surge suppressor or better yet get a UPS.

If you are still encountering issues I would recommend you visit an Apple Store to have them test your system. There are quite a few possibilities so one can't just tell you its this vs that.


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I had both PSU and logic board replaced by Apple. PSU did not solve the issue, logic board exchange, did. I now have my second iMac 2015 which is dead. Tried with a new PSU, dead. HAve to fork out a lot of $ for a used logic board and hopefully its not the PSU too, my devices keep breaking...

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Sounds like you need to review your power services. Here's my action list:

- Are you using a properly grounded outlet?

- Is the fuse panel properly grounded?

- Are you sure the ground connection is in good shape?

- Are you overloading the circuit (20Amp)?

- Do you have a good UPS?

- Are all of your peripheral devices with plugged into the same UPS or surge suppressor?

- Is your cable and phone lines properly grounded?

- Are your Ethernet hubs/switches plugged into surge suppressors or UPS's?


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