Screen Blacks Out After Fans Run at Maximum I inherited an iMac and it has an issue. It boots up fine but within the first five minutes the fans start to slowly ramp up to full speed. After about five to ten minutes of the fans running at full speed the screen goes black and I lose control of the computer. I cannot see anything on the screen, not even the cursor moving. I am also not able to shut down the computer using keyboard shortcuts. I have to hold down the power button.

The computer actually is still working to some degree after the screen goes blank because I have had a video playing from a USB stick and I could still hear the audio after the screen blacked out. I once had a Youtube video playing via the WiFi and when the screen went black the sound stopped.

I took the computer apart thinking that maybe it was full of dust. There was very little dust and blowing it out did not make a difference.

I have done a SMC reset and and a NVRAM reset and neither made a difference.

I installed a temperature monitor and the following is the output in degrees F a little before the computer goes to a black screen:

AC/DC Supply: 96
Airport Card: 133
Ambient Air: 75
CPU Heatsink: 88
GPU Die: 262         &lt;------I think that this is as high as the sensor reads.
GPU Heatsink: 262
GPU Proximity: 241
Hard Drive Body: 100
LCD Proximity 91
Misc: 100
Optical Drive: 99  

When the computer is running properly all four diagnostic LEDs on the logic board are lit. When the display goes black LED number 4 turns off leaving the other 3 on. Based on the Mac diagnostic instructions, if the fourth LED is off the issue is with the LCD panel or there is no video signal being generated.

I suspect that the video card is not working properly due to the high temperatures. It looks like the heat from the die is making it to the heatsink so I think that the thermal paste on the GPU is okay.

The computer is an iMac model EMC2134
OS X 10.8.5
24" Core 2 Duo with ATI Radeon HD2600 256 MB video card  

I also ran EtreCheck and it said that the self test passed. I can post the results if needed.

I would like to repair the computer even though it is old. I however do not want to bring it in to a repair shop. What do you think is the issue and what leads you to that conclusion?

Thank you,
jbr
https://www.youtube.com/watch?v=qXso8KFG78M

see post from tommyjames 337 half way down the page

https://www.ifixit.com/Answers/View/126496/Graphic+card+overheating,+CPU+fan+in+full+speed,+then+black+screen

"Okay you guys! After months and months of trying, I have found the true solution to the overheating graphics card. I was right with my initial hypothesis. It was indeed the heat sink.

I was mislead for months because the heat sink I purchased initially to test my theory was also malfunctioning in the exact same way. But after isolating literally EVERY part in the iMac, buying a new logic board, and purchasing 5 new graphics cards (without heat sinks) to no avail, I finally decided it must actually be the heat sink, and it absolutely is."