Chosen Solution
So to close this off, I managed to fix the iMac but there were two separate problems. The first problem, LED 2,3 were green, but not the others, was solved by reseating the RAM. I then tested using the display before seating the parts back in place, but when I put the display back after everything was screwed in, LEDs 1,2,3,5 lit up, there was no backlight but I could use an external monitor and see the apple logo in the display when I shone my camera flashlight on it.This became an Odyssey. LEDs 1,2,3,5 lit up when the Mac was powered, and LED 2 when power was connected but the machine was off. I replaced the display, logic board and power supply at the expected cost but nothing worked. A final suggestion from The Bookyard suggested changing the eDP cable, as it was the only common part left to the new and old components. And hey presto, the backlight worked. The say that the backlight failure and the eDP cable have been implicated before and in my case that was the cause.The LED diagnostics and in which order they start up didn’t match with what I observed or expected. The Bookyard said they hadn’t seen my sequence before, esp LED 2 being on instead of LED 1. Food for thought - and these guys were extremely helpful. I hope this helps someone else - and thanks to all those who suggested fixes.
You’ll need to go back to the CPU as it’s not seated correctly and/or the socket pins are bent. The iMac is a hard system to replace the CPU in, as the RAM cage and heatsink get in the way.
Hi! I got same problem, I have 3 diferent logic boards with Radeon Vega 48, all the same issue, led2+led3 and fan spinning. Ram, using default 4GB+4GB not starting, tested with 4x32GB Crucial compatible and same. If I do this with the default logic board, it works with the same iMac 2019 27". Did you use Command+Alt+P+R to reset the RAM? I have tested also with i9 9900KF and got the same results, can be possible that the i9 Radeon Vega 48 Logic Board matches the CPU? I mean, I bought months ago a fresh Logic Board+i9 9900K CPU and I replaced without any problem. This 3 boards comes with Radeon Vega 48, two for i9 and the 3rd for ani5 3,7Ghz, without CPU. I have replaced the PSU, the same, but when I fit again the original Logic Board (i5 3,5Ghz 580RX) systems starts perfectly, no issue, all leds working, I mean that Display and connectors are ok, does any one replaced their i9 Radeon Vega 48 CPU or similar?
