Chosen Solution
Replaced power supply and memory, but iMac 20 still won’t boot up. Only 1 diagnostic light
This sounds more like your drive has failed if you are seeing the screen and an folder icon is shown with a blinking question mark. Taking the display off will impact the onboard LED’s as the screen won’t vibe wired in so while LED #1 is lit it only tells us power is getting to the logic board, as you also altered the RAM we can’t rule out if the RAM is good of fully inserted. I would go back to the original RAM to see if things are working.
