MacBook Pro Retina 2012 - sensor issue
The temp sensor is in the track pad so if you choose to undertake this repair, this is the part your are going to have to replace.



The PN# for the track pad for MBP 2009-2012 is PN# 922-9063.  The good news is that it's under $30 at the time of this writing.

To change out your track pad, you will need to remove the battery from the top case assembly.  iFixit.com has an excellent tear down which details how to do this.

Note:  You won't have to completely disassemble the unit, but you will have to remove the battery to gain access.  You can remove it by gently prying it up with a plastic putty knife.  I have even seen dental floss used to slowly work the battery loose.



If you are not comfortable doing this repair, I suggest you let a tech handle the repair.

I was able to find a YouTube video showing the battery removal.  Once it's removed, you will be able to get a good idea of where the track pad is located.