MP 6,1 Mac Pro Trashcan won't power on

[naujoks]

My trashcan won't turn on anymore. Out of the blue.
Only the lights at the back come on, but nothing at all happens when I press the power button.
This is already the second trashcan I have that has the same symptom.
When it happened with the first machine I owned, I actually had another working machine that I could use to replace the power supply. In fact, I swapped every swappable part (CPU, RAM, graphics card, top board, bottom board) and nothing would change so the fault had to be on the mainboard. I was never able to pinpoint the problem though.
So what now? Is that problem becoming more frequent?
Any ideas what I could try this time? I don't have a second machine to swap parts anymore.

 

[macsound1]

Since there's lights on the back panel, atleast you know it has power.
Have you tried running a hardware test?
Unplug everything except the keyboard and display and hold D at startup.

 

[naujoks]

I have everyting unplugged. I tried holding D, but the computer doesn't even get to a stage where that could have an effect. It's not booting at all.

 

[th0masp]

It has diagnostics on the mainboard: https://logi.wiki/index.php/Mac_Pro_6,1_Trash_Can_Late_2013_Diagnostic_LEDs

There's a pretty in-depth Apple document for repair technicians working on this type of machine somewhere on these forums - I've seen it linked in multiple places. Maybe somebody can dig up the name/link.