Gas Fitter Practice Test

What is a common indication that a thermocouple has failed?

1. The pilot light stays lit

2. The appliance operates normally

3. The appliance fails to ignite

4. The thermostat adjusts temperature properly

The correct answer is: The appliance fails to ignite

A thermocouple is a critical safety device in gas appliances, serving as a flame sensor that ensures the gas valve remains closed when the pilot light is out. When a thermocouple has failed, it may not properly detect the heat from the pilot light, which leads to an important consequence: the gas valve does not open, and as a result, the appliance fails to ignite. This failure occurs because the thermocouple is no longer generating the voltage necessary to keep the gas flow operational. Thus, when attempting to ignite the appliance, the user will notice that it does not start, which is a clear indication of a thermocouple failure. The other options relate to scenarios where the thermocouple is functioning correctly or not causing a failure. For instance, if the pilot light stays lit or the appliance operates normally, it suggests that the thermocouple is working as intended. Similarly, if the thermostat adjusts the temperature properly, this indicates normal operation of the appliance. Therefore, recognizing the failure of a thermocouple surfaces predominantly through the inability of the appliance to ignite due to the failure of the thermocouple to signal the gas valve effectively.