iMac shuts down unexpectedly, only starts again with SMC reset

[grogthecaveman]

Hi, I have a problem, and I'm trying to fix it. I have an iMac on 10.10.5, and it passes the Apple Diagnostics. Yet it won't stay powered on for more than a few hours, but usually just tens of minutes. It will shut down on its own, with no warning, just as if someone yanked the power cable out. It does this whether it's in use, logged out, or even in Recovery mode. Safe mode doesn't help. Resetting PRAM/NVRAM doesn't help. Repair Disk Permissions doesn't help. I thought it might be USB related, but I've tried a couple different keyboards and mice in literally every combination of USB ports available (keyboard on 1, mouse on 2, 3, 4; keyboard on 2, mouse on 1, 3, 4; etc). I've even booted the computer with nothing plugged in except the power cable, and still, it shuts down on its own. Sleep mode has been disabled, but it hasn't changed a thing. I've tried it on different outlets and different circuits. I sent it in for warranty work, and they did hours and hours of stress tests, and it passed all of them. I get it back, and it shuts down on its own, just as before.

What else is there to do to fix this? Thanks in advance for any help.
[doublepost=1466787674][/doublepost]https://forums.macrumors.com/threads/imac-keeps-shutting-down-unexpectedly.1906058/

This thread suggests it's a faulty power supply. Is there any way to get something from the Console that might confirm this?

 

[chscag]

I've seen this before and it usually is an intermittent or defective power supply that's causing the shutdowns. Sudden shutdowns can also be caused by a bad temperature sensor but that is less likely than the PS. You can pickup a new PS from either eBay or www.powerbookmedic.com.

 

[grogthecaveman]

It's still under warranty. Is there any way to document when a power supply error causes the machine to shut down?

 

[chown33]

It's still under warranty. Is there any way to document when a power supply error causes the machine to shut down?

No. There isn't any mechanism to detect and log a power-supply malfunction.

In particular, iMacs don't have a power-fail monitor of any kind, that then signals the CPU to log a power-supply problem.

The best you might hope for is a subsequent startup will see the lack of a prior clean shutdown. The thing is, the lack of a clean shutdown can be caused by several things, such as simply pulling the plug out of the wall. It's not unambiguous.

If it's under warranty, take it in, tell them everything you posted here (i.e. print it out), and tell them how to replicate the problem. They may have test equipment that can monitor the power supply and diagnose in more detail, but that's up to them.

 

[chscag]

It's still under warranty. Is there any way to document when a power supply error causes the machine to shut down?

Not that I know of. However, if it's still under Apple care, take it to the genius bar and have them run extensive diagnostics on it. Tell them what you told us.