PC shutting down with no apparent reason during low load, does not respond to power button after crash.

[LowPolyRolyPoly]

Hello all,

I am having trouble with my PC shutting down. My PC is a Lenovo prebuilt, Lenovo T5 26amr5, with 5600g in a b550 board, 3060ti, 550W PSU. All major components are stock Lenovo OEM part. (CPU cooler, fan are aftermarket , and I have 2 HDD, one is a Toshiba P300, and the other is a shucked WD whitelabel.)

During period of low load,  my PC sometimes shut down without any warning. It doesnt go into BSOD, the signal to the screen just cut off, fan shutdown, HDD stop spinning. The only thing remains on is the front LED. There is no warning, but if it has unexpectedly turned off once during a day, it is highly likely to turn off again during the same day. This happen very randomly, so i have yet to catch it on video. This behavior have been ongoing for at least a year now. This never happened during my gaming session, when the machine is drawing lots of power, but only when i surf web or do something that is not demanding.

After crashing, attempting to push the power button to turn the machine back on have no effect. I have tried pressing it for 5 minutes straight to no avail. The only way to make it boot up again, is to unplug it from the wall, leave it there until the front LED is gone, then plug it in. After that, it will start up normally.

I have searched Event Viewer for any kind of crash log, but the only thing i see is an error log with event ID 41, Task category 63, and keyword (70368744177664),(2). There is no further info on the log, everything else in detail tab is either 0 or false. I have also reinstalled Windows several times, reintalled newest driver, but this keep happening. Temperature is sub 50c, so it shouldnt be a problem either.

Technicians from Lenovo have also replaced the Mobo twice, but it still crash. They are now telling me to send this in for repair, so i wanted to have some opinions on what i can do to diagnose this before sending it in.

What can i do to diagnose this further? What could be a possible cause for this? I appreciate everyone advice, as im at a loss of what to do as of now.

Thank you for reading this, and sorry for the new account. I only learned today that there is a troubleshoot section here.

EDIT: Tangentally related story of another solved problem: I once turned Enhanced Power Saving Mode on, which is fine at first boot, but on subsequent boot, reset power was tripped. I jumpstarted the PC with a paper clip, only for it to immediately reset power. Once i leave it for a half an hour unplugged, it turns on fine. It was fixed by disabling that setting.

[jsnotlout1]

The power issues almost make me thinks it's a PSU (power supply unit) issue. And the inability to turn it on almost seems like a safety feature. Maybe the PSU is shorting. Or the CPU is overheating due to poor Cooler contact. Those things can prevent it from turning on. You're best bet is probably sending it to be fixed. You could also try testing the ram. Ram can lead to all sorts of issues. Those are my only ideas. Hopefully one of the smarter members of the forum has some ideas.