OK, This is not going to be easy ;-[
The CPU riser board has a problem. I’m not sure if the sensor its self is the issue or you have a power feed connector failure which is overheating.
If you look at these two images you’ll see there is not obvious sensor around the slots (either side). I would have expected there would have been something more visible. What I do see is the power feed stand offs are near the 3 & 4 memory slots. So any corrosion or even a loose connection could cause the connection point to heat up thus the sensor of that side is going off.
[image|1538618]
[image|1538619]
Follow this IFIXIT guide to get the fan unit off [guide|21211] following through to Step8 then pick up from Step19 and 20 to check the power feed connections to the CPU riser board.
[image|1538630]
If cleaning the connections & screws using 85% isopropyl alcohol (and you may need a pink pencil eraser if you’ve got corrosion) doesn’t solve the issue, then you’ll need to take the CPU riser board fully out to inspect it. You might even need to replace it.