How to replace a thermocouple on a boiler

The No. 1 cause of pilot lights that won't stay lit is a worn-out thermocouple. It's easy to replace and a new one costs $5 to $10. Rather than trying to unfasten the thermocouple inside the water heater's burning chamber, it's easier to remove the whole burner and thermocouple assembly.

Bad thermocouple – A thermocouple is essentially a sensor that regulates the gas supply to the boiler allowing the pilot light to ignite. If your thermocouple is dirty or faulty, this can prevent your boiler from starting up and producing heat. Approximate repair cost: $350 - $550.

Failed Thermocouple Symptoms The first is signs of contamination on the tube, which can include discoloration, cracks, or pinholes. Next, check the wiring for any signs of wear or corrosion like missing insulation or bare wire.

When purchasing a replacement thermocouple or flame sensor, it's essential to make sure that it's compatible with your particular furnace model. Honeywell, White Rodgers, and other manufacturers make universal replacement thermocouples, usually with a 30-millivolts (mV) rating for standing-pilot furnaces.