Will replacing a thermocouple increase gas furnace efficiency?

If the thermocouple is broken, then replacing it will increase the gas furnace efficiency. A thermocouple is device that turns off the gas if the pilot light goes out. In the event that you replace a functioning thermocouple, then it is unlikely to increase the efficiency of the gas furnace. In general terms, a new component usually increases efficiency of the overall machine, but there is no definite proof that this is the case with the thermocouple and gas furnace efficiency. Whether the thermocouple is broken, or you are hoping to increase efficiency, make sure you turn off the gas line before replacing the thermocouple, as advised by the Fuel Gas Code, to prevent gas leaks.