My kid's boiler worked fine for about 10 years,l before the thermocouple quit. My back was really out of whack, then, so I got a furnace gut to replace it. Good for about a week.
Called me, out again, feeling a bit better, replaced it myself. ( I was IN the furnace business, 35 to 15 years ago, so was not , so not in over my ass. I Do know what I am doing.)
Lated a couple months, replaced, same, replaced, same. Place I used to get my thermocouples had a tester, Busy Beaver, HD, Lowes, don't. I went to a local dealer, closes at 5 PM, couldn't buy from them. Asked them to test the thermocouples I had replaced, and they were all "good", per the tester.
Why the furnace quit, I don't know. Why it worked when I replaced it, I don't know. I HOPE the one she had replaced without me keeps working. My local store told me that you do not want to replace with the "PlumbCraft" type, cheapos. The original, which I still had, was the brand that they sold, BUT, THAT thing lasted only about 10 years. Sealed unit.
The poster above is referring to a millivolt unit, electrical, NOT a tubular, capillary unit, it has wires leading to the gas valve.
I am beginning to think that my daughter's gas valve is the culprit.
My own gas boiler ran for over 30 years with the same thermocouple.
Cheers,
George