When a hot water tap drips or the heater fails to start, the culprit is often a defective thermocouple. This small, metal device sits on the flame of the pilot of a gas water heater and keeps the gas valve open only when the pilot is lit. If it fails, your heater can go cold or even pose a safety hazard. Being aware of the warning signs can help you detect problems before they become costly fixes.

1. Pilot Flame Keeps Going Out
The most obvious symptom is that the pilot flame keeps going out. A healthy thermocouple generates a small electric current when exposed to heat. That current keeps the gas valve open. If the thermocouple is stuck in a "cold" state, the valve will close, and the pilot will extinguish. You may find yourself repeatedly relighting the flame.

2. Heater Shuts Off While Water Remains Hot
Following the pilot ignition, the heater should stay on, heating water for 20‑30 minutes. If the thermocouple fails, the gas valve will turn off early, even though the water stays hot. You'll see the temperature fall suddenly, and the heater will need relighting every time.

3. Unusual Smells or Lack of Heat
A defective thermocouple can trigger the safety shut‑off, cutting off gas flow. This might produce a slight chemical odor from the gas or, worse, no heat. If you notice a gas smell or the thermostat reads well below the set temperature, examine the thermocouple.

4. Pilot Flame Flickers or Looks Dim
If the thermocouple is marginal, the pilot flame may appear weak or flicker. This occurs as the gas valve cycles on and off intermittently. The pilot may seem steady, but the control is actually unstable.

5. Inconsistent Water Temperature
If the water temperature swings erratically, from hot to lukewarm, the thermocouple could be miscalibrated. Even a small misalignment can result in the gas valve opening at wrong times.

6. Physical Damage or Corrosion
Look for visible signs of wear on the thermocouple. Corrosion, cracks, or a broken stem means it can no longer generate sufficient voltage. Long-term, the metal may degrade, particularly in damp or humid environments.

7. Pilot Light Lags Behind the Thermocouple’s Movement
Should you turn the pilot flame on manually and the thermocouple fails to respond quickly, it may be stuck. A delayed response usually means the sensor isn’t detecting heat correctly.

8. Error Codes and Warning Lights
Contemporary gas water heaters will often show error codes if the thermocouple fails. A flashing LED or panel code may indicate a "pilot light" or "sensor" problem.

9. Unusual Noise from the Heater
A rattling or clicking noise during startup can suggest the gas valve flicking, another symptom of a failing thermocouple.

10. Repeated Pilot Relighting by a Plumber
Should a plumber need to relight the pilot repeatedly in a short span, it clearly signals the thermocouple isn’t working.

Steps to Replace a Faulty Thermocouple
Shut off the gas supply and allow the heater to cool
Detach the old thermocouple using a wrench or pliers
Look for corrosion or damage; if present, replace it
Wipe down the connection points and 名古屋市東区 エコキュート 交換 pilot flame area
Install the new thermocouple, aligning the stem with the pilot flame
Reopen the gas, relight the pilot, and verify the heater stays on

If you’re not comfortable working with gas appliances, call a licensed plumber
Replacing it promptly can stop gas waste, restore hot water, and keep your home safe