Loud fan and CPU over utilized making the Mac Pro unusable.

Lets start off with getting a bit more diagnostic info. Apples service center diag report doesn't offer much help in figuring out what's failed. I find TG Pro can give us a bit more help. Basically it will show us if the sensors are even responding or not vs just a go - no go status in the report. Install it and also get the full version as the diagnostics might also help. Post a snapshot of the sensors and fan info you'll likely need to adjust the window size to get everything into one capture or take two making sure to slide the slide down to get all of the sensors post them here for us to see.

I'm assuming you've never cleaned the internals of your system. While the sensors are telling us its a bit warm inside we don't know if the fan is even spinning or if the blades are not fowled up with dust and junk (overheating). So as a matter of course I pop the fan assembly off and give it a good clean following this guide Mac Pro Late 2013 Fan Replacement for now we only need to get to Step 9 to gain access to assembly and then jump down to Step 14 & 15 to gain access to the blades to carefully wipe down with a soft paint brush and use a can of can'ed air with short blasts to blow the debris away. The fan should rotate with little effort. Reassemble and test the system again. Also tell us (picture) on how dusty things where.

Let's start there.