Bradford and White, 40 gallon, pilot issues

The unit is about 10 years old, looks good enough to warrant a repair, I thought. Pilot won't stay on, I tested the thermocouple, after 3 minutes up to .018v. My reading specifies 20 to 30 so I took it out, certainly looked old and scorched enough. I bought the Home Depot clone - Everbilt brand - put it in, it read 26 after 30 seconds. Hooked it up, the pilot stayed on first time, fired up, all done - this yesterday.

Today I got a text saying the pilot isn't on. Damn.

Did you clean the filter?

I didn't. The pilot stayed on just fine with the button pushed down but went off immediately when I let it up. From this is figured air flow was OK.

Also it all worked fine after I installed the new one but the cover had been off, that would have given new air into the chamber.

So the answer is no, I didn't clean it. Will do so.

Is the over heat sensor tripped? And, is it a self diagnosing gas valve? How old is it and is it open combustion or closed?

I should have checked the age. He told me 10 years, I should know by now people remember wrong. The serial number starts with CJ, that would be Sept '06, puts the age up to about 12.5 years. I saw a few rust streaks but no standing water. I looked again today, not sure how I missed it but there's no small amount of rust around the bottom. Not sure how it leaks then seals itself.

Doesn't have the heat sensor button, it's the older setup. It's open combustion. Given the rust and the age, we're putting in a new one tomorrow.

If it’s open combustion, it was produced before 2003.