Question PC randomly shutting down, no BSOD, lots of weird behaviour

Dec 5, 2019
Before I start: I am fairly sure that the problem is either the CPU or mobo but I would like to be able to confirm this conclusively.

What is happening: the PC runs for between 3-12 hours, and shuts down. It either crashes with the display still on, the CPU fan and one internal fan running (the other three are connected to the same header but do not run), PSU running, and an error code of A on the mobo. Or it just switches off. No load, just idle.

Behaviour when I try to start it back up varies hugely. It either doesn't start at all, it starts up and fails (with FF error code on the mobo), or it gets stuck on post with an error code of 19 on the mobo (Entry to CK505 programming, once I switch off and on again it works).

The most recent failure was very interesting: the display was stuck, I turned it off, it wouldn't start when I pressed power so I cleared the CMOS, it tried to start but shut itself off after a few seconds (FF error code). If I cleared the CMOS again, it would try again (and fail) but as soon as I stopped clearing it would shut off (i.e. it was affected by my clearing the CMOS as well as whatever is causing the problem).

I have been through a ton of stuff. GPU, cooling, software (I am getting nothing in Windows, no dumps, only kernel power errors), RAM (clear on memtest), and swapped PSU for a gold-rated 80+ that is working on another build. I have also checked connections on the mobo. CPU was overclocked but taking that off made no difference.

At this point, I can't see what else it can be apart from the mobo or the CPU. I have no idea which one though. FF suggests CPU but I have tested CPU under load, it is fine. But the errors really aren't consistent at all which kind of suggests that it isn't the mobo (presumably, if something was failing it would every time in the same way) clue.


PlexHd x79 Chinese knockoff board - I know but it has served me well for about 9 months-1 year, on pretty much all the time, I have another similar board that is great.
16gb RAM
EVGA W1 600W 80+ - also tested with EVGA 750W GQ though
1660 Super


Dec 31, 2015
If your motherboard gives you the option you could try under clocking the CPU and having it run a stress test for a while to eliminate the possibility of CPU degradation. I've never seen a CPU fail without physical damage. I believe you're correct to presume the motherboard is the issue