Random reboots with late 2015 27" iMac-Troubleshooting?

[camner]

I've had a refurbished late 2015 17" iMac since 2017, and for the last year or so, it has experienced random shutdowns. Without warning, everything will freeze (except for an external hard drive, which starts clicking away when the screen, keyboard, and mouse freeze), and then within 60 seconds or so, the iMac will reboot itself.

I looked at the log via Terminal, and the previous shutdown cause code is always -64. Looking via Google, it seems this code means that the cause of the shutdown is "unknown."

Suspecting it could be a bad memory module (I have 4 x 8GB = 32GB), I removed one module at a time and ran the iMac with 24GB, figuring if it was a memory issue, the issue would go away with the flaky module removed. But the random shutdowns persisted.

I'm not sure how to proceed with troubleshooting this. Any ideas?

 

[AlexGraphicD]

I've had a refurbished late 2015 17" iMac since 2017, and for the last year or so, it has experienced random shutdowns. Without warning, everything will freeze (except for an external hard drive, which starts clicking away when the screen, keyboard, and mouse freeze), and then within 60 seconds or so, the iMac will reboot itself.

I looked at the log via Terminal, and the previous shutdown cause code is always -64. Looking via Google, it seems this code means that the cause of the shutdown is "unknown."

Suspecting it could be a bad memory module (I have 4 x 8GB = 32GB), I removed one module at a time and ran the iMac with 24GB, figuring if it was a memory issue, the issue would go away with the flaky module removed. But the random shutdowns persisted.

I'm not sure how to proceed with troubleshooting this. Any ideas?

I have the exact annoying issue and I also made a thread on this forum. I tried all the troubleshooting there is, wiped the drive multiple times, clean install etc. but still have the issue. Do you notice high temperatures just before the computer freezes and restarts?

 

[camner]

Yeah, I've tried the clean install (what a pain!) also. And also without success.

Your question about high temps is interesting. I haven't noticed that, but I haven't specifically looked for that, either. I use TG Pro which turns the fans on more when the temps get hot, and I can hear those fans, so I'll be sure to listen for the fans running at higher speeds when the machine reboots itself.

Did you notice higher temps correlated with crashing?

 

[AlexGraphicD]

Yeah, I've tried the clean install (what a pain!) also. And also without success.

Your question about high temps is interesting. I haven't noticed that, but I haven't specifically looked for that, either. I use TG Pro which turns the fans on more when the temps get hot, and I can hear those fans, so I'll be sure to listen for the fans running at higher speeds when the machine reboots itself.

Did you notice higher temps correlated with crashing?

Yeah, kinda higher about 140 - 155 F average. That's when I've noticed the reboots happening almost always. I use iStat Menus to monitor temperature and fan speed.

 

[camner]

140-155 F isn't really all that high when the iMac is working hard. For example, when iMovie is rendering a movie I can easily get the CPU % to 100% and CPU temps to get in the 185 F range. At the point, TG Pro makes the fans rev to max speed. Interestingly, without TG Pro, the system itself NEVER turns the fans up very much.

I once heard someone speculate that high temps can cause permanent damage, but I don't know how reliable the source was.

 

[AlexGraphicD]

140-155 F isn't really all that high when the iMac is working hard. For example, when iMovie is rendering a movie I can easily get the CPU % to 100% and CPU temps to get in the 185 F range. At the point, TG Pro makes the fans rev to max speed. Interestingly, without TG Pro, the system itself NEVER turns the fans up very much.

I once heard someone speculate that high temps can cause permanent damage, but I don't know how reliable the source was.

But the thing is that, I get those temps even if all I have open is safari, finder and Apple mail. The temp levels are much lower when I turn on the iMac but as it goes on they get higher to that point, as I said even when I don’t do any intensive task. it doesn’t make sense.

 

[killerrobot]

Have you ever ran a Hardware test?

Use Apple Diagnostics to test your Mac - Apple Support

Apple Diagnostics, formerly known as Apple Hardware Test, can check your Mac for hardware issues.

support.apple.com

I've been experiencing the random freezes on my late 2015 27 5k, but it's never caused random shutdowns. I do blame Catalina for a lot of the problems because I feel when I updated to it that's when everything started.