In what scenario would you need to replace a thermocouple?

Replacing a thermocouple is necessary when the pilot light does not remain lit despite making adjustments. The thermocouple functions as a safety device, generating a small voltage to keep the gas valve open while it detects heat from the pilot flame. If the signal is weak or absent, typically due to a malfunctioning thermocouple, the pilot light will fail to stay alight as a safety precaution to prevent gas leakage.

In the context of the other scenarios, having a pilot light constantly burning indicates that the thermocouple is likely functioning properly. A successful pilot re-establishment test suggests that the thermocouple is also effective, having reactivated the gas flow upon satisfying the conditions. Steady high voltage readings during a closed circuit millivolt test imply that the thermocouple is generating an adequate voltage, further indicating that a replacement is unnecessary in those situations. Thus, the need for a thermocouple replacement is most evident when the pilot light won't stay lit, highlighting a failure in the thermocouple's ability to sense the flame or produce sufficient voltage to keep the associated safety mechanisms operational.