Unknown red light on logic board. Not DIAG RAM LEDs Mac Pro 3.1 (2008)

I have a Mac Pro 2008 with a solid/permanent Red LED on the logic board and RAM cards as soon as I press Power on. But, none of the existing diagnostics LED's seem to fit the problem it has.

The LED that actually is turned on is the D55 LED, beneath the 7 diagnostic LED.

Fans work but there are is no chime nor click.

This is putting a big question mark on what's happening as I can't see or read about anything related to this problem. I've done, PRAM and SMC reset, unplugged everything else, also changed the RAM modules, switched RAM cards, and put new modules in, exchanged the GPU and the problem persists.

Anyone had this problem? Is it the logic board that is going to waste? Or, is it the PSU?

Here's 2 pictures that illustrate the problem.

- Here's the exact red light led that stays on:

https://imgur.com/a/XBPuo

-Here's the led lights that lite on when pressing the 'DIAG' button:

https://imgur.com/IJb02jw

Any clues about what's happening? Would really appreciate some light on this.

Many thanks