Gas hot water boiler not turning on past pilot light and click on thermostat for more heat. The flame on the furnace does not turn on. Changed out the Thermocoupler, After starting the pilot light, initially the flames came on, but did not stay on. and never turned on again.
Thermocouples produce a minute amount of current with a minuscule voltage. The connection to the gas valve where the new thermocouple is installed needs to be clean and free of debris or it will not be reliable. Take the connection apart, carefully clean the contacts, and put it back together. That may solve the problem.