How do I know if my fireplace thermocouple is bad?

Author: Prof. August Brakus  |  Last update: Monday, November 3, 2025

Common Signs of a Failing Thermocouple Here are some common indicators: Difficulty Lighting the Fireplace: If you struggle to light your fireplace, or if it takes several tries to keep the pilot lit, it could suggest that the thermocouple is not sensing the flame correctly and is failing to keep the gas supply open.

How do I know if my thermocouple needs replacing?

The best way to verify that you need a new thermocouple is to see if your pilot light is lit. If the pilot light can't hold ignition, that means either a problem with the gas supply or, more likely, the need for thermocouple replacement.

Will the pilot light stay lit if the thermocouple is bad?

Thank your for your question. Yes, if the pilot light is not staying on then the thermocouple could have failed and needs to be replaced. But that being said, if the pilot flame is weak and is not engulfing the thermocouple, then the thermocouple would not be receiving enough heat to charge.

How much does it cost to replace a thermocouple on a fireplace?

Common Gas Fireplace Issues and Repair Options

Relighting is typically inexpensive unless the thermocouple requires replacement, which can range between $120 and $300.

Can I replace a thermocouple myself?

Here at Boss Plumbing, we highly recommend you not try to replace the thermocouple on your water heater yourself. Though not a highly complex water heater repair, it nevertheless does involve the use of gas. Thus, if you are unsure about what you are doing, one small mistake could lead to a disaster.

Fireplace How To: Testing A Thermocouple

How to tell if a thermocouple is bad?

If the pilot has a weak, orange-yellow flame or barely reaches the thermocouple, it's too low. If the flame extends well above the thermocouple, it's too high. Pro Tip: Test the thermocouple with a multimeter with a DC volt scale. If the thermocouple reads more than 25 millivolts, it's not the source of the problem.

Why does the pilot light on my gas fireplace keep going out?

If your gas fireplace pilot light keeps going out, you may have a problem with faulty pilot controls. The pilot control failing due to age or a mechanical issue can cause your pilot light to keep going out and prevent it from being relit.

What causes thermocouples to go bad?

Overheating: Each thermocouple is designed to operate within a certain temperature range. Operating beyond this range can damage the thermocouple's materials, causing them to degrade faster than normal. Contamination: Exposure to harmful chemicals or debris can affect the thermocouple's performance.

How do I know which thermocouple to buy?

The most commonly criteria used to make that choice are the temperature range, the chemical resistance, the abrasion and vibration resistance and the installation requirements. Installation requirements would also dictate your choice of a thermocouple probe.

Should the pilot light be touching the thermocouple?

The pilot flame should touch the thermocouple at a specific level. If the flame is blue and weak, and barely reaches the thermocouple, it needs to be stronger. If it extends well above the thermocouple, it's too strong.

What is the common problem in thermocouples?

Temperature sensor and thermocouple errors can be caused by various factors, such as mechanical damage due to physical stress, abrasion, or corrosion; electrical interference from nearby sources of electromagnetic radiation; wiring problems like loose, broken, or mismatched connections; calibration issues that cause ...

What does a worn out thermocouple look like?

Failed Thermocouple Symptoms

The first is signs of contamination on the tube, which can include discoloration, cracks, or pinholes. Next, check the wiring for any signs of wear or corrosion like missing insulation or bare wire.

Why does my gas fireplace go out after a few minutes?

Your gas fireplace could be turning off for several reasons. The most like culprits are likely tied to some kind of problem with your setup. Anything from issues with your pilot light, clogged ports on burners, damage to heating components, and thermocouple/thermopile malfunctions can be to blame.

How long do thermocouples last?

To determine if your thermocouple only needs a good cleaning or requires replacement, it will come down to how often your pilot light is going out. If you're experiencing pilot light outages every few days or have a water heater that is over 10 years old, you'll do better by replacing the thermocouple.

What is the difference between a thermopile and thermocouple?

While both devices are crucial for temperature measurement and control, their specific applications and capabilities vary. Thermocouples are primarily used for direct temperature measurement and safety controls, whereas thermopiles are preferred for applications requiring higher sensitivity and voltage output.

What are the symptoms of a failed thermocouple?

If the flame lights and goes out when you release the gas control knob after holding it in for the recommended 20 to 30 seconds, that's the sign of a thermocouple malfunction. Before jumping to the conclusion that the thermocouple is at fault, look at the pilot flame.

Can you clean a thermocouple?

Yes, you can clean a thermocouple, but the task must be done with a light touch. Remove the thermocouple from the process assembly, using a steel wool or emery cloth, lightly clean the tip removing any build up. Cleaning a thermocouple is not a fix for an issue, such as inaccurate readings.

What happens when a thermocouple goes out?

Without a thermocouple, it will seep tiny bits of gas into the combustion chamber of the device. In a few hours, it can leak enough gas for a small explosion, probably just enough to shorten your eyebrows.

Where is the thermocouple on a gas fireplace?

You will find thermocouples in both pilot light and spark start / hot glow gas controls. To avoid the constant pilot light draw on your propane tank, you should get a spark start or hot glow fireplace (or at least a spark start / hot glow gas valve) instead. Then there is no pilot light.

How to fix a bad thermocouple?

Once you know that you need a new thermocouple, here is what to do:
  1. Turn off the gas. ...
  2. Disconnect the thermocouple and two gas tubes from the valve. ...
  3. Remove the thermocouple. ...
  4. Buy a new thermocouple. ...
  5. Repeat steps 2-3 in reverse to install the new thermocouple. ...
  6. Turn the gas on and relight the pilot.

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