I went to the apple store with an issue where my IMac would freeze up, to the point where eventually disk utility didn't even recognize the main hdd. The genius at apple decided, through the extremely unreliable SMART diagnostics that the hdd was ok. He conducted a simple format and installation of the OS, whereby he sent me home thinking the problem was fixed. But it all happened again!
The repair was flawless, thanks to ifixit and their extremely simple instructions. It was easy and even fun!
Follow the instructions to the detail and make sure you know what screws go where.