iMac fans at max speed from power on, diagnostics error PFM006

So since today, on my late 2015 5K 27" i5 3,3 iMac (w/AMD), the fan runs at maximum speed right from power on, with a 10-20 sec delay or so, but way before anything is loaded. I use Boot Camp, too, and it doesn't matter which OS I boot.

I tried SMC/PRAM resets, didn't change anything.

No malware as per Malwarebytes. Also didn't install anything new recently. Activity monitor only shows the kernel_task, which AFAIK just is the fan maxing out and / or thermal protection.

Diagnostics gives me the error message PFM006. My suspicion was a faulty thermal sensor, so I installed iStat Menus and received the stats shown in the screen shot below.

When I try to change the fan speed via iStat, nothing happens, it stays at max no matter what I set.

Now all the thermal sensors seem to have normal values, far from hot. Only the AC/DC T2 Secondary Heat sink sensor doesn't show any data, but then my question is: Does my iMac even have that, and should there be data? In other words, have I found the error? If so, what could I replace? AFAIK, the power supply unit comes with an integrated heat sensor, but I'm not sure if that's the case for all models. If so, I'd try replacing that! Or what are your opinions? Anything else I could try?

[Re-Titled by Moderator]