Chosen Solution
Hello.. I recently attempted to replace the CPU and hard drive in my 27 inch late 2015 iMac (17,1). I replaced the cpu with a 6700k. I was careful not to touch the power supply coils. But regardless .. after reconnecting everything the power button does not turn on the computer. Of the 4 diagnostics LEDs , only the bottom led displays green when the power cord is connected to computer. When I press the power button nothing happens.. the fan does not move.. none of the diagnostic lights change. I was hoping it might be the psu so I replaced it with a new one from fixit.com but still getting nothing but a single green led on diagnostic board. I also tried removing and re-seating all memory sticks in iMac as well. I would be grateful for any advice or suggestions.
I would go back in and take the CPU out check the socket pins for damage as well as the chip. I’m suspecting you didn’t mount it properly, this is a common issue in the iMac’s as the heat sink makes it hard to properly align it and then the proper process of screwing the screws. Think how you tighten the tire rims lug nuts. You criss-cross the tightening so the rim sits firmly and flat onto the hub. Here we use the same approach! Slowly tightening the screws in the same pattern so the pressure on the chip is equal across. If not some pins won’t make contact, at worse the chip can be cracked!
