I recently disassembled one of my G5's (2.3DP) as the back side fan was running full tilt with the U3 temperature reading ~85ºC and a hardware test pointed to the U3 heatsink:
***ERROR*CODE*** 2STH/1/2: U3 HEATSINK
It was also in need of a thorough cleaning and was a good excuse to do a full tear down of which I had never done before with a G5 tower. My hope was that I would find the U3 heatsink out of place and I would also redo the thermal compound on the CPU's while I had it apart.
Many screws later, including some self taught disassembly instructions (how many models of this beast are there!?) revealed the U3 heatsink was still fully attached and everything looked fine except for some dried out thermal compound. I went ahead and replaced the thermal compound (thermal grizzly kryonaut), put some better nylon spring rivets on the U3 assembly for good measure, and also redid the compound on the 2 CPU's.
After reassembly, it booted up on the first try, but the fan came back in full force and the temperature reading for the U3 heatsink was still ~85ºC. Another hardware test also came back with the same logic board U3 error code.
I'm guessing at this point there must be a bad solder joint somewhere on the mother board that I couldn't see. Another observation is the side of the case is cool to the touch and doesn't seem even close to the reported temperature. Had a good time none the less and might just use the 2x2.3's in another G5 I have (2.0DP)... that is if they will run at full speed?
If anyone has other ideas about the error code, please feel free to chime in.