Baked GPU: Target Display Mode not working properly

Hi,

My iMac display died some weeks ago. It was a well known issue: several colored stripes on the screen (green in my case) and the iMac would not boot up.

After researching the problem I quickly diagnosed that it was the GPU that was failing. I found several posts online which suggest to try and bake the GPU in the oven for 10 minutes at 200 degree celsius.

While it was quite an operation, it worked. It took me about 5-6 hours to do. Fist opening up the iMac, removing the logic board, and finally removing and baking the GPU. I applied new thermal paste (Arctic Silver). Put the maschine together, and it worked, it booted again! I was so thrilled!

But then I tried to use the Target Display Mode to use the iMAc as a second screen for my MBP Late 2013, which has worked flawlessly last couple of years.

TDM works . The MBP sees the iMac. I am able to initiate with cmd + f2. The problem is that the display of the iMac keeps flashing/flickering and is not usable. I’m also able to use my imac to output sound, which sounds distorted/cracked. Yesterday, after some time, it even worked for some time, but after quitting/enabling TDM, it was broken again.

I’ve tried everything to get it back to work. Updating the OS’s, resetting SMC, VRAM etc this did not help.

What’s strange to me is that the iMac works perfectly (for as far as I can see right now). As a standalone computer that is. It boots, it displays, there is sound everything seems to work. Only TDM is not working.

I tried using Apple Hardware Test, but for some reason that keeps crashing on the memory test? I used 3rd party memory tests to see if that works, which it does. But AHT does not!

I just run Cinebench, which also worked and seems OK.

Is there anything I can try? I’m really wondering what the problem is because like I said, the machine works perfectly beside the TDM.

The only thing I can think of is the GPU failing somehow in TDM. I’m not sure how TDM works internally though. Buying another GPU might be an idea, but thats pretty expensive for this older machine (300 $ or so) and even with that it might not be te problem.

Thanks in advance.