Chosen Solution
My iMac’s screen suddenly started showing stripes only. It starts up normally, but I can’t see anything, Everything works fine but the screen itself. All it’s displaying is just stripes vertical and horizontal. The iMac never fell, nothing ever hit it. using another display works fine ! The display rarely comes back to normal after hours of working with my external display image But when rebooting the Mac it starts up stripped again . I think the display works fine since that it works from time to time, the GPU too since that mirroring works. But I have no idea what’s going on if someone can help please I napped leaving music on the computer, after waking up I found the screen totally messed up. I wonder if I over used it. I tried everything I can rebooting and so on, nothing works. Now I have no idea what to do! I’m a graphic designer and I work with it a lot. It never showed any malfunction for two years until now, I also have no warranty left and I don’t trust third parties at all here in Morocco, most are scamers.
If you setup your system in a dual display mode, that would imply the Sweep logic within the display assembly its self has a problem. For reference here is the back of the 5K model while the logic on yours is different it is basically the same: 27" 5K Display assembly logic board. I would take the system to an Apple Store for repairs as it might be covered under an extended warranty. Sadly you need a new display assembly.
