Greetings,
I have a Rheem 40gal gas water heater, approximately four years old have had no problems till today, pilot light will not remain lit, I suspect the thermocouple, I tried tapping on the gas valve while lighting, attempted five times, no success...

Hello Olio Funger,

You nailed it down yes it could be the thermocouple it is cheap and easy to change, good luck.

John

https://www.youtube.com/watch?v=jDvo4yJ1OdE

I will agree on both counts

I also agree, while it could be something else, that is where you start. As noted, they are low cost and easy to change.