A standard residential water heater thermocouple typically lasts between 5 to 10 years, depending heavily on usage frequency, ambient humidity, and whether the pilot assembly experiences soot buildup from incomplete combustion. Over years of continuous exposure to open flames and high thermal stress, the internal dissimilar metal junction degrades and gradually loses its ability to convert thermal flame energy into the required millivolt electrical current. Verification: Test your thermocouple's open-circuit millivolt output using a digital multimeter during annual maintenance checks.
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