Signs Of A Failing Water Heater Thermocouple

When a hot water faucet drips or the heater fails to start, the culprit is often a defective thermocouple. The device is small and metal sits on the pilot flame of a gas water heater and keeps the gas valve open only when the pilot is lit. If it fails, your heater can shut off or even pose a safety hazard. Being aware of the warning signs can help you catch problems before they turn into costly repairs.



1. Pilot Flame Keeps Going Out
The most prominent symptom is a pilot flame that keeps extinguishing. A healthy thermocouple creates 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 Turns Off Despite Hot Water
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 may result in a faint chemical odor from the gas or, worse, no heat at all. If you notice a gas smell or the thermostat reads well below the set temperature, examine the thermocouple.



4. Pilot Light Flickers or Appears Dim
When a thermocouple is marginal, the pilot flame may look weak or flicker. This is because the gas valve is intermittently closing and opening. The pilot may look steady, but the underlying control is unstable.



5. Variable 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. Wear or Corrosion
Check the thermocouple for visible wear signs. Corrosion, cracks, or a broken stem means it can no longer generate sufficient voltage. The metal can degrade over time, especially in damp or humid settings.



7. Pilot Flame Lags Behind Thermocouple Response
If you manually turn the pilot flame on and the thermocouple does not respond quickly, it may be stuck. A lagging response typically signals the sensor isn’t sensing heat properly.



8. Alarm Codes or 名古屋市東区 給湯器 修理 LED Alerts
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
Turn off the gas supply and wait for the heater to cool
Detach the old thermocouple using a wrench or pliers
Look for corrosion or damage; if present, replace it
Clean the connection points and pilot flame area
Put in the new thermocouple, aligning its stem with the pilot flame
Turn the gas back on, relight the pilot, and ensure the heater stays on



If you’re uneasy working with gas appliances, contact a licensed plumber
A quick replacement can avoid gas waste, restore hot water, and keep your home safe