IMAC 2011 27 black screen

[Gupmaster]

Hey guys, new to the forum and have done my best searching what the issues could be but hard to narrow down. So risked buying a 2nd mac which was fine when i got it. Updated os to high seirra and then started getting black screen of death. Worked on and off but now doesnt work at all. Just a black screen. So ive tried all the different boot key options and get nothing. I hear the chime on start up. If i plug in a external display i get a white display and thats it. I took the screen out to make sure all cables are connected and not damaged. All seen good. With display connected i can see diagnostic lights 1,2 and 3 on but not 4. This is as far as my knowledge takes me. Many thanks for any help and advice in advance

 

[Silencio]

The GPUs on the 2011 27" iMacs are notoriously fickle. There are actually quite a few good threads on here about this, including some non stock replacement options if you're feeling adventurous.