2017 iMac shutting down spontaneously

[DN667]

Since yesterday my 2017 iMac 5k / 27" shuts down at random moments. By shutting down I don't mean a clean system shutdown, but more like somebody pulls the plug. There aren't any power issues in my house; I even tried the iMac with an UPS, but to no avail.

The Apple Hardware Test (AHT / self diagnostic) tells that everything is fine. I've reset the SMC. Temperature seems to be fine as well. The system logs doesn't show anything interesting, they just stop logging at the time the system fails.

This issue hasn't been happening before - it started 24hrs ago and has happend 10+ times since.

Where can I look for a solution / what to do? Any tips are welcome!

 

[rpmurray]

When you start the iMac up does it give you the "unexpectedly shut down" dialog? And are you sure that it actually shuts down and it's not just the display going black or the iMac going to sleep? Have you checked the Energy Saver settings?

Have you made any changes or installed any new software in the last 24 hours?

Edit: When you say the temps are fine, how are you monitoring them?

 

[DN667]

Thanx for your reply - no,it does not display this message.

I'm pretty sure it shuts down, as e.g. the lights on my USB stick dim fully when the iMac fails. Energy saver is disabled.

For what software is concerned: no changes there. Perhaps an update of MS Office? Nothing significant though.

Temperatures: iStat

 

[ignatius345]

What USB peripherals do you have plugged in?

I had a really similar issue with my 2014 iMac 5K. Full shutdowns, no warnings. As you say, it was like someone pulled the power cord out of the wall. I think it was a USB hub I had plugged in, because when I removed that from the equation the issue went away.

 

[DN667]

The only USB device is my amplifier (DAC), which has been off for the entire time. I'll unplug it and see what happens, yet I wouldn't expect this to be the issue, as the amp has been connected non-stop for over 4 years.

 

[Fishrrman]

It could be a power supply failure.

 

[paul_bace]

Sorry to bring this thread up from the past, but currently sitting on a 2017 iMac that once warmed up (after about 2 hours of use), the machine will shut down, and won't restart unless I pull the power cable (SMC), and then it'll shut down right after I enter user password, until I leave it for an hour or so.

I've had the computer apart 3 times already (first to upgrade, then to diagnose), it's been internally cleaned. The SSD is 2tb NVME with adapter (which I've also replaced), the power supply looks good, doesn't smell or anything.

Everything seems to be pointing to power supply, but now weary that a "tested working" from eBay will have the same problem.

Any advice or info would be appreciated. Like is there anyway to make sure it's the PSU, is there any way to confirm a working replacement (from research it seems 2017-19 might be a bad year, but PSU's from 2012 onwards are all interchangeable).

EDIT: Also, installed WINDOWS to rule out software. Was able to play DOOM on high settings for a while, then just when I thought it was macOS, another random shutdown, and then the same as described above.

Temps never really go over 35-50 degrees.

Specs:
2017 27" 3.8ghz QC - 32gb Ram - 2tb NVME - 2tb HDD - RX 580 8gb

Thanks in advance,
Paul

 

[real_mon2]

Start with the simple stuff. Remove all power. Remove and reseat and removable ram modules. We had a client in with a similar fault. His IT group could not get the unit to power up. No power on chime. We carved open the 5k screen to observe the led boot pattern. Power supply was fine. Complained about the memory which we initially thought was factory soldered. After reseating the ram, worked fine.

While yours could be the power supply, it is not common. If you have a multimeter, we can review if you may have a possible shorted capacitor which is very common to motherboards. Over time, caps just due and act like a short circuit which will then consume too much current.

Try to boot with a SMC bypass to rule out a faulty sensor. This process will cause the fans to run at full blast.

For your model, how did you handle the hard drive thermal sensor with your SSD?

Just some thoughts to consider.

 

[paul_bace]

Start with the simple stuff. Remove all power. Remove and reseat and removable ram modules. We had a client in with a similar fault. His IT group could not get the unit to power up. No power on chime. We carved open the 5k screen to observe the led boot pattern. Power supply was fine. Complained about the memory which we initially thought was factory soldered. After reseating the ram, worked fine.

While yours could be the power supply, it is not common. If you have a multimeter, we can review if you may have a possible shorted capacitor which is very common to motherboards. Over time, caps just due and act like a short circuit which will then consume too much current.

Try to boot with a SMC bypass to rule out a faulty sensor. This process will cause the fans to run at full blast.

For your model, how did you handle the hard drive thermal sensor with your SSD?

Just some thoughts to consider.

Turned out to be the power supply. Also, what sensor do you speak of? I broke up the fusion drive, and put and NVME in the PCIe slot. Thermal sensor on the 2tb HDD was still there, if that's what you mean?