three thermocouples in a year

Water heater, maybe 15 years old. pilot out, replaced thermocouple. all good, except maybe half a year later, same thing. relights, but goes out as soon as i release override. replaced thermocouple again, stays lit. very short lifespan for a thermocouple, but should be good now. 6 months later, same thing. thermocouple again fixes the problem. if it was the control valve, curious why the thermocouple would solve the problem. but why would a thermocouple go bad that quickly? and then again, roughly same interval again? does a new thermocouple somehow temporarily "trick" a defective/dying control valve? did i get a batch of bad thermocouples?

yes, tip fully inserted into housing. exactly where it's supposed to be. yes, nut tight at control valve.

no, i didn't charge for the last two trips out.

????

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