No Internal or External Display or Boot Chime - Screen Share Works
I recently replaced the LCD and upgraded the hard disk to an SSD on my 2010 21.5" iMac. After installing the SSD, I installed a clean copy of macOS on the disk. For the first few days after the upgrade things were going well. Then the the screen would begin to flicker, eventually going dark. The end of the video cable looked worn out when I put the new LCD in, so I figured that was the problem. I replaced the video cable and the screen worked for an hour or so before going black again. Diagnostic LED 1, 2, and 3 are on. LED 4 is off. Despite having no screen, I can use Screen Sharing and SSH to log into the machine. Using Screen Share the computer is working fine. When I add an external display to the computer, the external display is dark but the Screen Sharing software recognizes this change and expands the desktop width.
When the computer starts there is no boot chime. I did reset the PVRAM and SMC. When I reset the PVRAM there was a single chime.
Using the iMac 2010 Apple Technicians guide I believe the problem is either with the GPU or the Logic Board. In 2013 the GPU was replaced by Apple, so the GPU currently in the computer is not the original one. I would like to hear an experts opinion on what the problem may be.