I'm beginning to think that the "MB" sensor in PC Probe (or alternatively, "System" in SpeedFan) isn't the northbridge at all (at least, it's not actually near the NB).
I wasn't too satisfied with the stock cooling on the P5Q Pro, so I pulled off the heatsink assembly (the northsink + heatpipe + MOSFET heatsink). At first, I just put on a HR-05 IFX. No difference in temperatures, despite the new heatsink being barely warm. Today, I finished installing a HR-09 on the west side MOSFETs (the ones in front of the rear I/O ports) -- instant 20C-30C drop in temps on the motherboard sensor.
I guess it might work ok as a northbridge sensor, since on the stock cooling, the heatpipe is connecting the MOSFET and NB heatsink...
EDIT: Well there goes that theory -- the temps are back up. Oddly enough, the whole time I was running OCCT, the sensor was at room temperature. Now that it's idle, it's climbing back up. Eh, I've just about given up trying to figure the sensors on this board out. The CPU sensor is just flat out hosed and usually reads over 120C, although there's supposedly a new BIOS that fixes that. The motherboard sensor, I have no idea what it's the temperature of. Oy vey...
EDIT: Oh great...now the sensor is the exact same temp as my video card reading in CCC :roll:. Damn it ASUS, why do you enjoy confounding me?
Just finished up a round of CoH, and now it looks like both sensors are hosed and are reading >100C. CCC reading is still steady at 56C, so I guess it's not GPU temp either.