Chosen Solution
I tried to upgrade the CPU on my imac 2009 27" (EMC 2309). It was a blunder because after I put back everything, the screen goes black within a few seconds after power on. Sometime the screen stays until the login window appears, sometime during the boot progress bar, sometime just right a few seconds. I can still connect to the imac from a laptop. When I plug an external monitor to the imac, the login window appears on the external display as usually. I thought it’s because of the new CPU, so I put the original CPU back. But the problem remains after that. It seems I broke my imac? Is there a way to find out what is broken: the logicboard, the graphic card, or the board on the display?
A system that winks out quickly could be many different issues and the hardest to nail down without using spares to swap in. What happens when you try booting up in Target Disk Mode? Restart and press the T key to enter. You should see a FireWire Icon on the screen does it stay visible? Reference: Mac startup key combinations If it stays running then you’ve proved the discreet GPU needs to be looked at. If not then you need to look at the thermals as the system is likely sitting down from a heat build up. That could be the thermal paste and pads on the GPU as well as the CPU and the systems logic board or displays thermal sensors, or a bad fan . There are other possibilities besides thermals but given the system was working? Before you went in that’s what I would focus on.
