1546. When and How to Reset a Mac's SMC and PRAM

My Mac has been performing strangely lately--among other things, the fan runs all the time, even when the computer's not doing anything. Do I need to replace it?

This is such a common problem it is worth re-visiting even if we have discussed 
it before.

Your Mac contains some non-volital memory. This memory doesn't lose the values even when you turn ythe computer off. And it controls the behavior of your Mac. It sometimes can get a little messed up, maybe some memory got written to that shouldn't have ot something is just wrong. And it is easy to reset these areas of the computer.

Fix 1/2 in a gap.

http://www.makeuseof.com/tag/reset-macs-smc-pram/