Repair iMac 2011. White screen?

Question

Do it yourself

Hello, owner of the animal Imac 21.5 mid-2011. At a perfect moment, the picture disappeared from the screen.
The Internet sentenced me to contact the service. A bug was fixed in the power harness service, now when the computer starts, the screen at least glows (before that, the matrix did not start).
The service offered to change the video chip or wait for a donor to check on it. Since I had to wait a very long time, I took the computer from the service in order to try to repair it myself, including performing a reball.
All this background, now to the question. All 4/4 LEDs are lit on the diagnostic LEDs, and a white screen is lit on the screen. It seems like I understood 3 LED says that everything is fine with the GPU and CPU. Tell me how I can test the circuitry, piping and loops. What are the typical problems with such diseases.
PS: Arms are straight. The service was adequate.

Answer 1

90% of the video card died, this is a weak point for poppies in 2011, it was exactly the same for me, the warranty ended a long time ago, but it turns out that Eppal, as always, has a secret program for out-of-warranty replacement of a video card, which is almost nowhere mentioned, and even after 6 years I changed it for free to off . video card service. Now I think this promotion has already ended, but it’s worth finding out on the Internet, because even at that time they told me by phone that they had not heard about such a promotion. If they refuse, look for a donor on Avito, but there are not many chances. Donor video cards are very scarce, and most importantly, the donor will also burn out, as Apple officially changes to an already normal chip with a good heat sink.