Well, by what you're saying.... do you have any random spare videocard to try in place of the one now installed ?

And do you have any card installed aside the gpu ?

Different monitor?

Reseat monitor cables.

So the Dell tech had you remove the CMOS battery, but did the person have you put a new one in?  They last around 5 years.  Once bad, will cause all sorts of seemingly unrelated bizarre problems.

@bradthetechnut  hardly the issue. He said they changed the mb, after, but it didn't fix the issue. So it's caused by either an addon card ( gpu etc ) or the psu

@mazzinia,

But did the new MB come with a new battery, or was the old one transferred?  I'm sure that's what you're referring to.  It's a cheap fix even if by chance it doesn't work.

I've seen PC's not boot or boot loop all because of the battery.  It can also make some downloads difficult.  Not that it needs changing every time as they last 5 years, but it's also best to have a good battery when updating BIOS.

The MB purchased I'm sure isn't brand new.  With one refurb I bought, first thing I had to do was replace the battery.

If BIOS can't be accessed on an older PC, you guessed what to check first.

I totally agree, but he mentioned a Dell Tech , so it means he's under warranty and the mb was replaced under warranty by Dell. 

It can be new or refurbished, that's up to Dell sending out what they think is more convenient for them, but in theory should come with a new cmos battery.

You're right too.  New board should technically have a new battery.

Follow up.

The machine has a thunderbolt card, plus 4 nvme drives, and a spinning drive, in addition to the gpu card, and both CPU sockets are populated.

Dell replaced a total of 6 components, before the machine finally would successfully post.

First the motherboard was replaced.  No change.

Next the graphics card and PSU were replaced were replaced. No change.

Next one of the two CPUs, plus 4 of the 24 DIMMS were replaced. The machine then worked, after some coaxing from the tech (repeated power-ons) , entered service-tag, and ran diagnostics. The tech chose one CPU to replace at random, same with the (ordered pairs of) DIMMs - i.e. the technician did not have any information to help him decide which to replace.

I'm surprised by how hit and miss the process was. I'm less confident about Dell's ability to service their precision hardware as a result of this experience. It would be helpful if the technicians could attach some device to the unit before post, to be able to quickly and reliably detect the failing components, but perhaps that's some wishful thinking.

After the motherboard was replaced, I now must re-run the DCU to get current BIOS, and re-run my tool to ensure that linux boots from nvme drives, instead of Microsoft Windows booting from spinning disk.

Well, that's quite the ordeal, and definitely thank you for documenting it.

I wonder if it was just the ram ( and the cpu was to play safe )

I'm curious if the Power Distribution Board was also replace when the PSU was replaced? I'm having similar problems with a T7920. Mine the screen stays on and locked, no USB; No Hard drive lite. So to me this indicates NO power to motherboard/CPU. The video card get the power from the Power Distribution Board so it's on, as the PSU is obviously on, but no power to the Motherboard? The computer restarts as soon as the power button is pushed and then all is well till the next lock up that is intermittent, maybe later on in the day. Can the Power Distribution Board be the problem?