If the CPU that you are using to test the boards is a later model and the BIOS on the board is an earlier one that doesn´t know it it´s possible the board won´t start.
Or did the boards come with CPUs?

They came with CPUs - with P4P800 I also tried an older P4. With P5PE - seller put P4 so it should work on all BIOS versions - also checked Celeron D.

I began to wonder if this may be an issue with BIOS - both mobos had their batteries removed for a while - and I've read some information about faulty BIOS happening. I may try to get an adapter for my programmer and reprogram it. But I wonder if it's worth spending time on already lost case.

I've read much about P4P800 sudden death, but not much about P5PE.

I think I've got a pattern here. Today I've acquired MSI 865pe neo2 (was inside a case I bought) and it has the same issue.

I've read more about it - in most cases the same pattern - computer works fine, until something is changed/replaced/moved/touched. It's like something stops connecting inside it - NB? And it seems to be always Intel 865 chipset - no matter if socket 478 or 775.

One day maybe someone will find a solution or I'll reprogram BIOS and it will be the cause.

Plug a PS2 Keyboard in and hold in the DEL key before you power on, keep the key pressed!

See if you get a post.

What PSU are you using to test these ? Those older soc 775 require a ATX12v spec of 1.1 or higher but may not start with anything after 2.01, have had issues with some older boards when using a too old or too new ATX12v /EPS spec PSU.
The newest ATX PSU ATX12v spec may not even power them up, had that happen with a 848 chipset board, with a newer PSU (ATX12v spec 2.3 EPS12 2.9) would not turn on, had to use an older PSU....just a thought.

Hah, I was able to run MSI 865pe neo2 - seems it has only dirty/faulty RAM slot - after cleanup and putting into different one it boots up.

But both ASUSes - still nothing - tried also DEL key and PSU specifically marked as for P4 (and confirmed working with MSI board).

Horun wrote on 2021-01-26, 03:03:

What PSU are you using to test these ? Those older soc 775 require a ATX12v spec of 1.1 or higher but may not start with anything after 2.01, have had issues with some older boards when using a too old or too new ATX12v /EPS spec PSU.
The newest ATX PSU ATX12v spec may not even power them up, had that happen with a 848 chipset board, with a newer PSU (ATX12v spec 2.3 EPS12 2.9) would not turn on, had to use an older PSU....just a thought.

This is the first I've heard of this. If this is widespread I kind of regret giving away some of my older power supplies now. I stockpiled "refurbished" (used but clean) Seasonic 350ET and 550HT units and only kept a couple of older power supplies.