Sta Rite SR400NA heater is about 3 yrs old. started once this spring and ran for a couple hours to heat pool slightly. 3 months later and trying to start it up and it will not do anything. read out says 126.

This is a common problem and easy to fix. Your temp sensor screws into the front side of the water manifold. You will just get a new one and swap it out. Your book will show you just where it is, but it's the size of a 1/2" plug on the top front of the manifold when you take the cover off the heater. Good luck.

Here is copy of the user's manual for these pool heaters. You will have it just in case any other issues arise.

http://www.backyardcitypools.com/images/bwp/Max-E-Therm-Manual.pdf