A Google search shows that you are not alone in this. No-one seems to have found out what the auxtin sensor represents (the best guess I can find is that it is the Northbridge/Southbridge temperature). I'd ignore it.
If it's steady at 128, then that sounds like an artefact (128 being binary 1000000 it could just be a stuck bit in a register or sensor). If you Google "auxtin" and look at the results you'll find people have all sorts of odd temperatures for this reading (one reports 878 F!) but the general opinion - no-one knowing what it means - is that it is meaningless.
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