Still won't boot

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So here's a head scratcher.
Imac 2011. Power off issues lead me to replace the HDD to an SSD, the PSU and the logic board. They were cheap and I like the machine. Now, it ran for 2 days straight and suddenly won't boot. I got it to boot to Diagnostics for an hour and then won't boot no matter what combo of power cord. I'm like..is it the power button?? Or is there a PCB that also needs replacing? Does the GPU need it? I'm in so deep that I kind of want to finish it if possible.
 
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From a cold start, exactly what happens when you press the start button. Don't hold it down, just a quick press. Describe visual and sound.
 
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sure. One LED on. Left fan begins blowing. Second fan begins blowing. Then a few moments later both go nuts with blowing. One LED on. I have 2 PSU's to attmept with.
 
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I was really interested in the display activity. What did or does it do.
Alrighty, it does nothing. Display does nothing. LED's are unlit, except for the frist one. New info: My power button sort of passes a continuity test. Not sure how reliability matters for this. Then, everytime I plug it in the fans kick into overdrive. So, logic board seems to be evaluating sensor infrmation. But second green light doesn't come on. I have 2 power supplies here, I suppose they could both be faulty, but PS at rest shows 12 and 4 volts in the right spots. Once on I'm getting 12's and 3's. Maybe that's too low at 3. Finally, since it is my second logic board I lean toward it's not the logic board.
So now my questions (unless suggestions reveal otehrwise) are, 1. how to test the Power supply. 2. What constitutes a faulty button? 3. If the button is intermittent, would it prevent a boot, or cause power off events?
Anyway, th'at smy life haha
 
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This may or may not help but this is what I did.

2011 27" iMac.
At boot up it chimed but would only display a white screen After lots of reading other posts, I hand no reason to suspect the power supply or 3TB HDD. To me the symptoms were more like GPU. I removed it and cleaned up the old thermal grease and oven backed the PCB for ten minutes at 200c. While I had the mother board out, I removed the CPU, cleaned up the old thermal grease, (no oven bake), Put new thermal thermal grease on both GPU and CPU. Reassembled, booted and it worked like new. Been running perfect for two years.

2nd 2011 27" iMac
At boot up, black screen and fans worked. So why not give it the same treatment? I did. Both GPU (with oven bake) and CPU cleanup.
Booted and it worked well.

You can't lose any more than what you have already lost but I would do all this with the original GPU, CPU and drive.
Hope you can retrieve something from this process.
 

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