shutdown due to overtemp

Hi--

This afternoon I had one of my ES40s shutdown with an
overtemp error.  If I enter RMC I see that Zone0 is
49C whereas all my other zones are measuring 28C-- I
also noticed the system is giving a 12V bulk warning
on boot.  From what I have read Zone0 is a sensor located
near PCI slots 1-3 and Zone1 is slots 7-10 and Zone2 4-6.
Could the sensor just be bad or dirty?  Do I need to replace
the backplane?  Where is the sensor so I could try cleaning
it?

My reading also indicates that if RMC env command reports
all is well powerwise-- and the SRM show power command 
says things are ok that I need to replace my system board.
Has that been others experiences also?

Thanks for your insights!

Stephen