Plumber, plumbing, Water Heater Repair : Thermocouple Replacement. Upon arrival the customer stated that their water heater wasn’t working. Upon inspection I found that the thermocouple had failed. Replaced the thermocouple, tried to light pilot but it wouldn’t stay lit. Decided that the gas valve wasn’t working properly. Water Heater Repair: Gas Valve Replacement Upon installing a thermocouple and it still wasn’t staying lit, I decided that the gas valve had failed. Replaced gas valve, lit pilot, waited 25secs, pilot stayed lit, then turned gas valve to the B setting. Checked for proper burn, but since the basement is being remolded, there was some yellow flames from the sawdust floating around. Water heater is working properly now.