Chosen Solution
I would start off replacing the Power Supply. There might be more damage… iMac Intel 27" Retina 5K Display Power Supply Replacement
I was surprised the PSU was not the fault. Next, let’s take a look at the RAM. First we need to verify if it is the correct RAM for your machine, please pull it and tell us what you have.
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.
