Mac Book Pro Overheating
As Rob said, your lid switch might be the problem.

I would suggest following steps:


Reset your SMC and the PRAM/NVRAM.
Run Apple Hardware test to see if it comes up with anything.
Check your Pmset -g profile.


FYI:

There are 2 small magnets mounted on the lid (along the top edge)

You can find them by using paper clip. 

On the opposite side (of closed lid) there are 2 sensors, that normally should last forever (unless you put a strong magnet on them so they are stuck now), but then there is nothing that does.