You can determine that a thermocouple is failing if the pilot light repeatedly goes out shortly after you release the control button, or the main burner refuses to ignite. When the thermocouple's internal bimetallic junction degrades, it fails to convert flame heat into the millivolt electrical current needed to keep the gas safety valve open. Verification: Test the thermocouple's output voltage using a digital multimeter set to millivolts; a healthy unit should read above 20 millivolts, while a failing one reads below 12 mV.
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