bios temp vs mbm temp reading

soyo px400 2.4 celeron The bios reports temp at 45c and mbm at 31c whats is going on and what do I trust

For each given reading MBM has a choice of sensors from which to read.
I suspect that you need to change which sensor MBM is getting it's chip temp from.

I would think that bios and software would be the same since they use the same sensors. I have a thermal probe and it's always a little different than software (to within 1-2C). I would check which probe you're looking at on MBM/BIOS.

"I would think that bios and software would be the same since they use the same sensors."

Actually for my mobo (A7N8X) and probably a few others, bios temps are read from the socket sensor while the software might be measuring socket and/or diode temps. For example, bios reads my cpu temp as 22C and case as 16C. MBM measures cpu (socket) as 22C, Diode as 18C, and case as 16C.

If you are sure that you've setup MBM correctly and that your mobo is supported, I see not reason not to trust MBM. MBM might just be using a different sensor. As you can see my diode temp is lower than my socket/bios temp.