The primary signs of a failing or defective thermocouple include the pilot light repeatedly going out shortly after you release the control button, the main burner refusing to ignite even when the pilot is lit, or the water heater frequently losing hot water. If the thermocouple's internal copper conductor degrades, it fails to convert thermal flame energy into the millivolt electric current required to keep the safety magnet open inside the gas control valve. Verification: Test the thermocouple's open-circuit voltage with a multimeter; a healthy unit should read above 20 millivolts.
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