Question PC randomly shuts off then won't turn back on


Jul 27, 2016
CPU: 6700K
Mobo: MSI Z270 M5
PSU: Corsair HX850i

About 2 weeks ago I came back from a holiday and tried booting up my PC. It didn't do a thing. I knew it wasn't likely to be a PSU issue as the clear CMOS light at the back of the case was lit up however I still spent a few hours testing the PSU on another machine where it performed perfectly. I tried unplugging components but still nothing. I then decided to remove the CMOS battery and replaced it with a new one an hour or so later. It booted up. So 2 weeks pass and I'm playing a game when it just randomly turns off, then on about a second later, then off and does this about 10 times with varying times between before it stops, powered off and refuses to boot just like before. I removed the CMOS battery like before, waited about 20 minutes, put it back in and it stayed on for maybe 10 minutes before it did it again. Usually I love a challenge like this, finding the issue and fixing it but I'm now just extremely stressed with exams coming up and my code on the PC, I feel like I've tried everything and I'm just stumped. Any help at all would be massively appreciated.


The fact that "lights" work does not, in ANY way, mean there is not a PSU issue. There are separate rails and circuits on a PSU and ANY one of them could be failing while another is ok. 3v, 5v and 12v are all distinctly separate, and in fact, even on any one of those circuits there could be a single PART of them that has failed. We sometimes see just a single connector fail for that matter. Besides which, those lights use such a small amount of power that they could never be indicative of a "good" unit to begin with.

Your phone could power those LEDs.

The fact that it DOES work with another system however, debunks that probability, at least at THAT time. Now that it is not working again, I would again either try it with that other system again, because you might have been looking only at a unit that was fail-ing at that time, but worked when fully reset, but now, has fail-ED, and no longer works.

Or, if you have access to a volt meter or can purchase one, you can simply test the unit itself.

If the power supply is good, then it is MOST likely that you have a faulty motherboard. There is not much else that would cause this other than the PSU or motherboard.

If you are plugged into a power strip or UPS battery backup, remove that from the equation and plug directly into the wall socket. These are things known for also creating this kind of problem.