RE: Dell t5500 workstation diagnostics lights 2 3 4 blinking no display
Is your power light Amber or Green? Pb7 is for Amber power light.
Noticed power light is Amber in your video. Sorry, only resolved case I could find is my prior link, where it was CPU related. I'd take a magnifying glass to CPU socket to look for any debris, broken CPU capacitors, etc. and/or look for debris on riser connectors.
Here's another post of same pattern, no resolution unfortunately.
What Xeon and thermal paste are you using? Is CPU and heat sink making good contact (thin, spread out paste when heat sink removed)?