800 G1 SFF Random Shutdown and Power-on

[tcgreenaway]

Hello,

My EliteDesk 800 G1 SFF (product No. G5R47UT#ABA) randomly shuts down, and then reboots. But it's stranger than just that, it also powers up from completely powered down, as part of the insanity. I have to say I'm pretty much out of ideas.

Here's what I know/have tried:

- Windows 10 64-bit, latest updates implemented

- 16GB RAM, Samsung EVO870  SSD installed, no errors reported by Samsung drive software 

- v02.78 (latest) BIOS installed, flash was successful

- System passes multiple runs of BIOS diagnostics

- SFC reports no corrupted files (although a handful have more than one owner)

- Fast boot is disabled in Windows and the BIOS (in the BIOS, Clear Memory is selected)

- New power supply (pulled from another SFF but unused) installed

- BIOS watchdog timer is disabled

- Network and PXE boots are disabled

- Windows wake timers are disabled

- New motherboard (from AliExpress, but behaves and looks completely legit) swapped in in desperation, same processor installed on cleaned heatsink with new thermal compound, same behaviour

- CoreTemp reports CPU cores in the 30C degree temperature range unless I HeavyLoad the CPU, at which time the cores slowly increase in temperature to 75C

- CPU fan and PS fan are both working normally

- No overclocking (ever)

- Windows Defender on and offline scans report no viruses

- New CMOS battery and CMOS memory clear done

- System will shut off by itself after 20-odd minutes if left in BIOS setup

- No attributable cause for the behaviour, apart from possibly Windows 10 updates

- Behaviour is identical whether the system is powered on, or restarted from Windows

- It's not an instant shutoff. Windows is clearly indicating something is calling for a shutdown "Shutting down..." - I can even cancel the shutdown if I have unsaved work, although I can't hold off the inevitable shutdown for long

- I can't find a relevant community posting about the issue that contains a solution I haven't already tried (see above)

Does anyone have any ideas? As I used to hear on one of my favourite childhood TV programs, "It's driving me batty".

Thanks to all who have gotten this far, and cheers,

Terry

[tcgreenaway]

Updating this posting, in case it helps someone.

After discussing the situation with a local tech, I disconnected the HP power switch and swapped in a different momentary tact switch, left the two system LEDs which are part of the switch assembly unconnected, and recreated the single jumper on the header (the jumper pins on the header are not documented as far as I know, so I decided I'd better make sure it was there). I was lucky to have a bag of switches and cables I picked up from a computer recycler, so this was dead easy - for those without cables, it will be tricky to do this without shorting out pins on the header.

Success so far. The system, which was so flaky it wouldn't even power on properly before rebooting, is stable and running fine.