AO Smith Hotwater Heater Pilot
 

I have an AO Smith hot water heater model (GCV 40 100) that was mfg 10/08/2004 but was not installed/used until 2007.
The pilot extinguishes when the water reaches temp and shuts off.
It can easily be lighted and remains lit until the next cycle.
I have removed the burner assembly and cleaned, but same condition exists.
There are a series of parts that make up the burner assembly but I don't want to start replacing if this is not the problem.
Any help would be appreciated.

Stonerod