iMac display won't turn. Boots up on ext. display fan at full speed

Hi,

I bought a second hand iMac that is the exact same one I already have and took the display off from it to replace mine that got broken.

However, after being reconnected, the "new" display still won't turn on on my "old" iMac. What is weird is that if I put back the display in the iMac I just bought… it's not turning on anymore whereas when I received this used iMac it was working perfectly.

I checked the screen with a torch to find evidence of an image to determine if the backlight was damaged, but there is no image.

I carefully followed the ifixit tutorial to remove the screen and I was gentle in every step of it. Nothing got broken, torn off, bent or badly handled. The LVDS connector appears to be in good health, no bent pins, was not torn or bent (I checked with a microscope).

Some additional details:

The iMac is a 27" from 2017: A1419 EMC 3070.

On the 5 diagnostics leds on the mother board, the first three light up completely, the fourth one is lit, but dimly, which I think indicates there is limited (or only one way) communication between the motherboard and the display. The fifth led is completely off. The state of the leds is the same wether I plug the display in or not.

If I plug in an external monitor, the iMac boots up, but is in safe mode (fans at full speed, kernel process taking more than 200% of the cpu cause it cannot find the display and the temperature sensor…).

I tried with 3 different LVDS connectors and the power connector seems in perfect shape.

I bought ANOTHER display (same serial number and intended for my iMac) that was working perfectly and installed it to no avail.

I brought it to Apple but they do not want to look into it. They can only offer me a display and motherboard replacement for 1400€.

Anyone have any technical insight on what I could do or check?

Steve