Why is my iMac not responding (Black screen)

Goodmorning,

This IMac is giving me a load of problems. Most of the time it gives me a black screen. if i power it off and turn it back on- sometimes it’ll display a image, sometimes it don’t and when it does display a image, it can’t be put to sleep because it won’t wake up

I’m Diagnosing it as i type. Since i started working on it, i replaced the RAM, LCD, Logic Board, Video card (3 times), and HDD. it’s still giving me the same problem after all the replacements. ONLY THING I HAVENT REPLACED IS THE POWER SUPPLY

Diagnostic LEDS:

the IMAC will always show the First LED. The second LED will always show when turned on but if it gives the black screen..the LED lights stop at the first two, the other two doesnt show and that’s why i replaced the video card several times but that’s not the problem.

So I’m stumped here and would love some help. I’m guessing it’s the Power supply since that’s the only thing i haven’t replaced…maybe the PS not giving enough voltage to power on the IMAC fully causing it to behave erratically but I’m tired of throwing money at it so i want to make sure