Chosen Solution
It’s an iMac 8,1. Sensor says the GPU is overheating (about 15 - 20 Celsius above what it showed until now) but it’s actually not, it’s actually really cool when you check the back case with your hand. What might be overheating a bit is the power source, but this is located way above the GPU. I did not pay attention to the PS temperature, before, so not sure if it is overheating, but it’s the only thing that is warm on the back of the iMac, GPU is cool, SSD is cool. Ran an apple Hardware test and got this: 4MOT / 1 / 40000003: HDD- 1499 I did a search on that and found this: iMac Intel 20" EMC 2133 and 2210 Hard Drive Fan Replacement Why am I getting an Apple Hardware Error 4MOT/1/40000003:HDD-1364 Is there a 3rd HDD fan under the mobo as it seems, but not very well presented in the last 2 steps of the first link? By the error code above does it mean that the HDD fan under the mobo is failing and that’s why the PS is getting hotter? Why does the GPU sensor says it’s overheating when it’s not? Update (01/22/2019) One thing I noticed is if the screen gets put to sleep, after a while, the temperature is much lower. I use iStat, you have a screenshot with the sensors, would TGPro give any other info besides iStat ? Here are the temps on the “normal setting”, you have a screenshot of the fan speeds at that setting as well. Another thing that is weird is if I crank the fans does not seem to make much of a difference on the GPU temperature, unless it’s 90 Celsius or so, bringing it down to about 80. Cranking the fans makes a big difference on all other components. So basically, GPU just sits at 80 no matter what fan speed, if I open multiple youtube windows it goes to 90, and will come back to around 80 if fans are cranked.
Update (01/22/2019) Here you TG Pro
Update (01/24/2019) The readings
I would first check to see if the hard drive sensor is correctly attached to your hard drive (it is just stuck on there) and that the sensor cable is fully inserted into the logic board. See steps #13 & 14 of this guide: iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement
To start with the temp at the GPU is not the same as what you would feel on the external case. Think of it this way… Your oven is maybe warm to the touch but the you won’t touch the heating element inside! Lets backup here as you told us how much higher the temp went but you didn’t tell us the start point: “Sensor says the GPU is overheating (about 15 - 20 Celsius above what it showed until now)” Can you tell us how you measured this and if this was just one time jump. If it’s happening more often what are you doing when it happens. The AHT error 4MOT / 1 / 40000003: HDD- 1499 is telling us the hard drive fan has failed. So you do need to replace it following the guide you’ve already found. The HDD fan is mounted under the logic board which is why you need to lift it off. There are three fans in this system: CPU/GPU, HDD & ODD. As for why the power supply is getting hot and why you have a hot GPU will need further investigation. How about installing a good thermal monitoring app like TG Pro so you can see more effectively whats happening and when. Post a snapshot of the main window showing all of the sensors so we can see things too Adding images to an existing question It also sounds like you’ve done a bit of work in your system how about given us some history what you’ve done. I would also recommend you check your connections in case you reversed one (sensor or fan) as they are located next to each other and look the same. Update (01/24/2019) At this point I don’t seen anything with the GPU other than the diode sensor. I would dust the system out fully and clean both the CPU & GPU of the old thermal paste and pads and putting fresh paste and pads on. Given the systems age I think thats all thats required here.
