How do I fix my Mac Pro (2009) that won't boot up?

Caps lock only becomes available at about the same time as the login screen, if you are getting that far.

Almost everything USB comes off chips the backplane board. So if USB's are dead, it is not good sign. Look at the system Block diagram in the service manual for assurance on that. But backplane board failures are rare.

That said, used backplane boards are available. Just be sure to get the one that matches your ORIGINAL Mac-Pro model. I don't think you will need to update the firmware 4,1 > 5.1 again regardless.

Another place to measure Voltages is on the DVD drive power connectors. You should see +12 and +5 on those.

Wow, that's a lot of power fluctuations!

I call that tall board in the back the backplane board, because it mainly holds ALL the slots PCIe and SATA. It also holds the big chips that drive all the peripherals, but NOT the Northbridge chip itself, which is on the Processor shelf.

Mac Pro 2009-2102 models, as you know, the majority of the heavy duty logic is on the slide-out processor shelf. If yours seems to be OK, then the backplane board is likely the culprit, given your Extreme power situation.

About the simplest thing your Mac can do is when you hold the Alt/Option key at startup. it draws a discernible Gray screen, then over the next several minutes, adds an icon for each potentially-bootable drive it can find by brute force. When it has completed a pass, the wait cursor turns into an arrow pointer.

No drives need even be present in the machine for that much to work, and the code is all in the Mac's ROM. if it lights up the screen, but does not find a potentially-bootable drive, that is a separable issue.

your Mac has error correcting code RAM, and if not working properly, the power-on light would blink a code.

what was and is on the screen?

is the power light on solid, or blinking a code?

can you tell whether startup is proceeding?

is the caps-Lock light available?

did it ever work after firmware upgrade to 5,1? even once?

if the PRAM backup battery were dead, often the fans would not even come on for more than an instant.