128C temperature sensor on heatpipe outgoing air.

I’m suspecting the SMC chip has some corrosion so as the system heats up the sensor input fails so SMC pushes the Fan to the higher RPMs in addition the CPU’s clocking is also lowered as it wants to protect your system from over heating.

At this point you’ll need to remove the logic board and inspect it throughly on both sides. Follow this guide MacBook Pro 15" Retina Display Mid 2015 Logic Board Replacement and take note of any corrosion, discoloration, staining or visible damage on any of the parts. If you can take some good pictures and post them here for us to review Voeg afbeeldingen toe aan een bestaande vraag

Generally the board looks clean I don’t see any obvious issues which is a good thing!

You’ll need to get a bit deeper here as the images are not at the needed magnification to see much. I’ve marked up your images which you need to look deeper.

Often times I find going over the solder joints can catch a cold solder joint. Do you have access to a hot air workstation? If you do place some flux on the joints and reheat them. If you don’t you’ll need to find someone with the proper tools to rework the logic board. Don’t try to do this with an iron as all you’ll do is make a mess.

Here’s the areas

and the SMC its self if the upper two doesn’t fix things.