Chosen Solution
iStat Menus shows the temperature sensor of GPU die is not working. It wasn’t the case before disassembly. Where is that sensor located? I changed the thermal paste on GPU and CPU and replaced my HDD with SSD. But it is not an SSD related problem because when I plug back in HDD fans are still spinning at the highest RPM.
The reason was to replace HDD with SSD, renew thermal paste on GPU and CPU. I get this when I run diagnostics https://imgur.com/a/JaGzQPo
Update (01/27/2019) Would you please check these images out and tell me if you see anything wrong?
What did you do to your system? Your system has an onboard GPU chip. The sensor is within the chip. Are you sure iStat showed it? Not all GPU’s offer an onboard sensor. Update (01/28/2019) I suspect you have two different issues! First your system uses a custom 3.5” HDD so if you swapped out the drive for a SSD you’ll need this OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later Thats the cause of your fan running hard. As far your GPU: One of the micro SMD components on the die carrier may have been damaged in the process of cleaning and applying new thermal paste or you used too much so some of the thermal paste which leaked onto the die carrier altering the electrical properties of the micro SMD’s Update (01/28/2019) I do see this area thats damaged:
Compared to a good board
Based on the photos, something is off with the paste. It shouldn’t look like it overflowed as in the photos. For example it should not be on the green part of the board. Either you used too much or applied it too thin and incorrectly. Too much paste is actually very bad and can overflow and definitely overheating. Too little paste and manually spread thinly over the chip is also bad. Recommend you remove the paste and start again. First, do not use anything that will cause static electricity. That will kill your parts. Ground yourself with a grounding wrist strap and de-engerize yourself before you touch any electronic parts. Apply absolutely fresh paste to chip. Use no more than a pea size drop. Put drop in middle of chip. Do not spread around. Push thermal sink/fan gently on top of CPU chip so the contact pressure gently disperses the paste. Not every inch of the chip has to be covered. Your fans are kicking on or revolving very high to save your CPU from burning up. The CPU sensors on the logic board are reading high temperatures. Another thing to check is the BIOS and/or firmware version. Update to the latest. Hope this helps. I have been building, fixing, and updating computers for close to 30 years. Good luck!
