Have similar problem. I thought it was my i7 cpu, but cpu works perfectly at another machine.
I give 12v to motherboard with my bench psu (cooling was off).
I found that the led start blinking WHEN i press cpu at the top left (around pin 221). I saw at boardview that this side is for communication with ram. With another cpu i have the same problem. I put a piece of paper to cpu pins to isolate the problem.
Even with the ram dimms off, the problem exists. Ram slots seemed perfect. Cpu slot pins seemed perfect.
edit: Possible some secondary voltage was short. Found a low resitance (can’t remember how much) at PP3V3_S0 that i didn’t like, i inject 3v to check for hot areas, didn’t found.
Then i assemble machine, boot it and works.
Tried Apple Service Diagnostics, passed all tests.
Hypotheses:
It’s magically fixed (temporary or not)
It’s something missing (still i didn’t find it)
Blow the sd reader slot for dust, i might remove some piece of wire stucked within.
I did something without my knowledge that fixed the problem.