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Hi, I did it twice before, replacing faulty hard disks in two iMacs, everything went ok, this 3rd machine (only one normal 3TB HDD), i just swapped the HDD with SSD (Samsung 860 EVO) following the guide, with the frame and heat sensor, upon testing I can hear the startup sound but the screen showing just strips, from the rubbish I can sense the system is booting from internet or just flashing folder, the usual sign that no operating system, I double checked the cables and can’t find anything wrong as far as I can tell, but I admit the data cable (the wide cable with lock) may be pushed hard when trying to insert it back, did I damage the screen or the board?, please advise… I tried to remove and reinsert the cables but nothing changed, it is as it is… Here is a preview:

Please accept my question and help me, I have no clue without your answer… Update (04/27/2019) Hello guys and thanks for your responding, but it seems that there are some conflicts between my updates and your answers, so here is what I did… after I swapped the HDD and fall into the screen problem, I swapped back the original HDD that has mac os on it and boot the iMac, the same screen issue continue but this time after the initial boot, the screen went black completely with no back light at all… so I connect the iMac to external Display (my Samsung 50” TV) and I can see the output but it is continuously flashing on & off, the keyboard, the mouse and the system is responding but the flash never stops… I tried to search in the internet and I notice that many places point to the cable as a suspect so I connect another iMac using the same cable and the output was fine without any flashing… so the iMac display has the problem shown above and the output to the external monitor is flashing continuously… the iMac is working, programs and responding but the display is showing as above during the boot after that it turns off when the mac os reaches the log on screen, on the second monitor I see everything but with continuous flashing… I double checked the display cables and the sockets on the board and they are fine with no visible marks as far as much as magnified as I can tell… where does this put me… I appreciate any information to lead me, should I give up or there is something I can replace for sure… thank you all Update (09/18/2019) Dear @danj , @docmofix here is something new and I didn’t expect it by any means… today I received another 27” iMac, Late 2012 (EMC 2546) (with a dead hard disk) which is compatible with mine Late 2013 (EMC2639) and I connected the display from that machine to mine (which is suppose to have a problem in display), to my surprise the screen also shows the same symptoms as mine… I also connect my screen (which is suppose to be faulty) to the other iMac and it works perfectly… So it is not the screen at all, my screen worked perfectly on the other iMac, also an external HDMI monitor connected to my iMac works perfectly which means ?? where does that puts me? what part of my iMac does not work?

Sadly your display got damaged. During the the process of taking it off you likely damaged it. its very easy to damage if you don’t use the correct tools and follow the technique as outlined in the guide iMac Intel 27" EMC 2639 Display Replacement and here’s the needed part [linked product missing or disabled: IF174-035-1]. Presently iFixit is out of stock here’s the Apple P/N 661-7169. Update (04/28/2019) Onboard LED diagnostics

Here’s a breakdown of the onboard diagnostic LED’s:

  • LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.
  • LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
  • LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.
  • LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

I have found some things, that might help, you should try those: Disconnect all peripheralsRestart in Safe ModeRun Disk UtilityReset NVRAM (or PRAM)Power up your Mac with the Startup Manager and select your Startup Disk ManuallyLaunch Single User or Verbose Mode with TerminalReinstall macOS or OS XRun Apple Diagnostics or Apple Hardware Test

Hi @atheerioy, I’m having the same issue but still don’t understand the root cause. Have you been able to figure out what was happening and hence fix it? Have you been able to fix it? I have exactly the same issue. Also the 4 LED’s are ON Many thanks in advance for your help.

I have the same problem on a late 2012 27” iMac after a SSD install. On this one, after the screen is blurry like that, it switches resolutions down to 720p. Then that is the only resolution it will display. The problem is a faulty display connection. It could either be the display data cable or the socket the cable plugs into. The socket itself is easy to break tiny (not easily observable with naked eye) pieces off and get them stuck between pins, meaning the cable will not fully seat or will not make a connection on all pins. I could only see it with camera zoom on my phone. Broke off a corner

Piece of metal in connector

Anyway, try replacing the $20 display data cable. Or try getting a magnifier to look at the connector on the board. It wouldn’t seat at all until I got the piece out. Now it does the behavior I described at the beginning, including the blurry screen. It seems to realize that the display is going wrong though and drops to 720p. So subsequent boots are no longer blurry like that but really pixellated from too low of resolution. I have to reset nvram to get it to try the higher resolution again. I’m afraid I will have to replace the board where the socket is. This appears to have been a really common problem even fresh from the Apple Store. As evidenced by this compilation of questions about the same thing. The owner of this iMac tried to get the failing hard drive replaced at the Apple Store (after warranty), but they refused to do anything requiring the screen to come off. I should have taken this as a hint not to do this repair. They probably already knew from warranty work that these are very fragile to work on. Update: I tried replacing the display cable but same effect.

Has anyone found the solution for this? I do not believe the issue is with the screen/ connectors or anything related. I have replaced 10+ SSDs on these machines with no problems. When I boot off the original HDD there is no fault. However when I boot off the new SSD the screen becomes pixelated. I will try another SSD and see if that solves the issue. If anyone has found a solution please let me know!

it is not related to the ssd, I put back the original HD, I started without HD and no difference at all, eventually I found another another similar logic board from a faulty machine and the HD and ssd worked with… the only two reasons I can come up with are: I some how damaged the board accidentally, a scratch here or there…a matter of static electricity…