Chosen Solution
I’m trying to fix an old iMac that suffered from a regularly appearing black screen problem: after some hours the display would shut off. So far, I diagnosed that one issue was a defect HDD fan, and I replaced it. After I took it apart and put it together, the iMac shortly worked , but then had the same problem again after a period of 1-2 hours. Now, it completely fails to boot and just shows a grey screen after the password entry. The same happens when I try to enter the local or internet recovery modes. However, the hardware test is functioning normal and does not find any issues. As the iMac had a black screen issue before, I’m inclined to think that the graphics card was the issue all along and now gives up entirely as soon as the iMac requires some more intensive work to be done (such as drawing the normal UI). Another option would be that I messed something up during my repair work of the HDD fan and now I’m looking at two hardware problems. What is my best guess here? Could the grey issue arise from a defect graphics card? Or is a symptom for another well-known issue?
Lets see if we can focus the problem down. Are you able to boot your system in Target Disk Mode? We don’t really need to plug your system into another system for this test as we only want to see if the display is able to show the FireWire / Thunderbolt icon floating around. Does it? Reference: Mac startup key combinations Let it run this way does it stay working? If so the systems graphics card has failed. What is happening is the CPU’s intergraded graphics services are kicking in and giving you the display of the icon’s unlike before.
