Windows 10 shuts down for no reason

Windows 10 Pro on a home built machine. Has been operational for a couple of years. Started randomly shutting down a few weeks ago. And I mean randomly. The PC can be in sleep mode and wakes up just to shut down. No new programs installed, AFAIK virus free. Memory, CPU and GPU tests all OK.

Installed Shutdown blocker to intercept the shutdowns. Clearly something sends WM_QUERYENDSESSION. Interrupting the shutdown doesn't lead to any problems while I continue to use the PC. Network, performance, apps, all work OK.

No errors in Event Viewer, no tasks that I can see asking for the computer to shut down and no tasks that I can see have failed as a result of the interrupted shut down.

How do I figure out who sends the WM_QUERYENDSESSION?

Cyanna said:

Windows 10 Pro on a home built machine. Has been operational for a couple of years. Started randomly shutting down a few weeks ago. And I mean randomly. The PC can be in sleep mode and wakes up just to shut down. No new programs installed, AFAIK virus free. Memory, CPU and GPU tests all OK.

Installed Shutdown blocker to intercept the shutdowns. Clearly something sends WM_QUERYENDSESSION. Interrupting the shutdown doesn't lead to any problems while I continue to use the PC. Network, performance, apps, all work OK.

No errors in Event Viewer, no tasks that I can see asking for the computer to shut down and no tasks that I can see have failed as a result of the interrupted shut down.

How do I figure out who sends the WM_QUERYENDSESSION?

Could be as simple as your Power/On off switch (usually momentary on) and/or related wiring to the Motherboard.
Unplug the two pin header and power on over LAN (Wake on Lan) for a few days and see if it resolves the issue.