Replace the CMOS battery...

Mi Problema

While you have it apart, replace the CMOS battery. It is 5+ years old....

I used a Duracell CR2032.

Mi Solución

Fairly easy, have some superglue or a hot melt glue gun to secure the temp sensor retainer to the hard drive.

Mi Consejo

I used a two terabyte Western Digital Black drive as a replacement.

First I used an external case to temporarily hold my new drive, and then installed a fresh copy of Mac OS X El Capitan. I then used Migration Assistant (starts automatically) to move all the data over.

Then I installed the drive.

Look carefully on setting the display and front bezel back.

If it does not sit flush, you are pinching a wire, and need to lift up and look.