Here's a cut-and-paste from answer Computronix, one of the moderators here, gave in a similar situation:
"In this instance, Aux is simply an unassigned thermal input, which will typically read invalid values. When a hardware platform has no unassigned inputs, then Aux will instead read values from thermal sensors for memory, northbridge or southbridge chipset temperatures.
In Hardware Monitor, if Aux is being read from an unassigned input, then just ignore it. In SpeedFan, Aux can simply be disabled, and therefore ignored. My Aux temperature is currently 17c, which isn't possible, since ambient is 25c. I've seen it vary from -19c to 127c, which means it's an unassigned input from the super I/O (Input/Output) chip on the motherboard. It's a common anomoly."