Chosen Solution
Recently, iFixit had diagnosed an issue with the 2011 iMac as being the dreaded GPU fault. Instead of replacing or reballing the card, I bought another used unit and switched drives, and I was back to work. I have since replaced the iMac with a Mac Pro, sold the replacement iMac and wanted to try and repair my original iMac. I did the disassembly and followed a guide to “bake” the video card. It seemed to work as the Mac now booted up to a stable desktop. To test the repair, I left it on for 3-4 days and it would always come out of sleep. I noticed that there was a software update, so I did the update (High Sierra 10.13.6). Since this update, the machine will boot up to the desktop, work fine for a few minutes, then get a sleep window (randomly) and turn off. Before the “bake”, the iMac would get about 50% through the startup, present a solid gray screen and re-boot automatically in a continuous loop. Is this just a different symptom of the GPU video card or is this another isolated issue as the machine now completes the boot cycle? Any suggestions on more in depth diagnosis?
What is happening here is the POST is not completing. Give this a try: Restart your system and hold the T key to enter into Target Disk Mode. If your system boots up with the FireWire/Thunderbolt icons floating on your screen, you’ve proved the GPU is still failing. TDM leverages the built-in GPU services within the Intel CPU, it also bypasses the dedicated GPU POST testing.
Does cpu fan working fine any heating issue may cause this reboot. Refix ram or other hardware. I dont this due to update it reboot.
Hi Dan and imac Einstein After pressing T button after startup, the thunderbolt icons drifts on screen. My inquiry: (1)is GPU the only problem? (2)Replacing is the only solution? (3)Is there any way to know how many hours the GPU has been in service after manufacture? (4) why GPU cannot be fixed by parts replacement? (5) during safe mode, why the mac looks normal without auto restart? (6) can upgrading to more advanced ios such as Monterey, or downgrade to lower ios , solve this auto reboot issue? (7) if i buy a new gpu, is it better to buy 1G instead of 2G to reduce attrition and extend service lifespan years?
