Chosen Solution
My friend sent me an iMac that won’t boot up, no fan, no hard drive movement. I examined the power LED, even the power is plugged in, none of the LED is on. Then I replace the PSU. I thought the problem is the PSU. Here’s the funny thing: after I replace it, the power LED#1 flashes once when I plug in, then nothing happened afterwards. No LEDs, no fan, no movement. What could go wrong? Here’s the VIDEO
Sadly you have a logic board failure. It could be more localized to the GPU board if the board is shorted out. For most people replacing the full logic board is just the easier path. Here’s the IFIXIT guide: iMac Intel 21.5" EMC 2389 Logic Board Replacement and here’s the needed part: iMac Intel 21.5" EMC 2389 Logic Board While I love these older systems as they are more repairable than what Apple is selling now you are going to hit the issue of Apple moving on with their OS. Mojave is not supported on this system as the GPU logic does not support Metal services needed in Mojave. While Sierra and High Sierra as still supported keep in mind in a few years they will be end of life’d and no longer have security updates.
