Tuesday morning I had no hot water. Go down to basement, water heater pilot is out. It relights without any trouble. Stayed lit all day. Next morning the same thing. Relight it and go to work. Mrs BASE texts me that she has no hot water. So now wife is unhappy...I make some time to look at it.

Pulled it apart cleaned air inlet and thermocouple. Put it back together and check thermocouple output. No problems there. Seems okay, I cycle it on and off a few times with the thermostat. A few hours later no hot water. It went out again.

I bought a thermocouple (actually pilot burner assy on this one) on the way home but I'm pretty sure that's not the problem. It was an expensive SOB so I think I'd rather return it that install it for no reason.

Is there anything besides the valve that could be causing this? I'm kinda stumped here.

Its a State (AO Smith) GS6 50 YBRT if that matters. Older style mechanical valve. I'm thinking it's probably about 8-9 yrs old so maybe it's not worth fixing.

Every time that happened to me it was the thermocouple.  I usually just scrubbed it with 400-600 grit sandpaper and it would stay lit until it needed done again.

gonna be thermocouple or the main valve. has to be one or the other

I know, that's been my experience as well. I guess that's why I stopped and got a new one before I even really looked at it. It seems to be working fine though. I was getting around 30mV out of it when I tested it. I did give it the sandpaper cleaning treatment as well.

That thermocouple has a temp cut out mounted inline with it. I guess that could be failing. Does not seem likely though.

I could install the new part just to rule it out. Stupid thing cost $89...that's the only reason I didn't throw it on anyway.

Off to price out white-rodgers control valves and new water heaters...

Yeah, you are right. The only other thing I could think of was airflow. Birdsnest in the exhaust, or dust in the intake filter. I did just cut up the basement floor to run a new drain so there is lots of dust down there. All vacuumed out and it's as clean as the day it was made right now and still not working.

Screw it. I'll put the new thermocouple and pilot assy in. And then I'll go buy a valve and put that in too. Tank will probably spring a leak as soon as I'm done spending money on it.

Well I think we figured it out. Actually Mrs BASE figured it out while I was a work yesterday.

This goofy thing has a ceramic disc that sits under the burner. It has a bunch of tiny holes in it that the combustion air has to enter through. I thought I had cleaned it well but this thing was filthy underneath. Hard to get to and even to see as you've got to go through the tiny hole for the air intake filter. We cobbled together some tubing and brushes and cleaned it the best we could last night. Had a hot shower this morning.

I guess when these newer ones aren't drawing air properly they have a thermal cutout that shuts it all down. From what I've read the stupid ceramic disc is a safety feature to keep it from igniting flammable liquid that might get spilled under it.

I had no idea any of that crap was in there. I miss the days when everything wasn't so over complicated with safety shit that causes more problems than it solves.

I had a similar problem and vaccum under and inside the was always the answer.

Yeah the wife was searching the internet and saw plenty of others were having that issue.

I never would have thought of it. That silly disc thing seems like a horrible idea but I'm sure it's save some idiot from burning his house down at one point.

I had a town house that if you ran the dryer it drafted enough to kill the pilot.

The most common cause is the 'thermocuple.'
Often a liquid filled capillary tube and NOT an actual thermocouple.

https://en.wikipedia.org/wiki/Thermocouple

One last follow up.

It's been a few days now and the thing is still running fine.

Cleaning that flame arrester, or whatever it is, seems to have fixed it.

Since the thermocouples cost so much for these, it's definitely worth trying this before you go out and spend and money on it. The part that gets dirty is not something you would normally ever see as it's hidden up under the burner, but keeping it clean is apparently very important. I'm not sure if this is unique to the State or AO Smith heaters but it's worth checking out before calling a repair guy or buying and parts.

They are not that bad if you shop around.

I have a spare extra long 'universal' one in the drawer.

I have another used one from a tank that started leaking.

The gas valve is in there also.  Those get expensive.

Had the same problem with our water heater about a year ago, just over a year after we installed it new.  Finally tried wiping the air intake vent holes on the bottom, stayed lit for a year.  Then it went out again, so I wiped the underside, and it has stayed lit since.  That was about a month ago.

Rust forms from the moisture running up the vent (and baffles) through the middle of the typical tank.

It falls down on the burner.
The gas pressure is not high enough to blow it off at that point.

It can block enough tiny orifices to both put out the pilot, block the secondary flame, or not allow it to ignite the larger burner.

The gas valve will only allow a short period with the 'flame proving' of the capillary tube heating before shutting down.

The tiny pilot fist igniters the 'flame proving' orifice (you will see the pilot more than double in size) and only then is the main burner provided with gas.