What Does a Thermocouple Do on a Water Heater?

No hot water again? While this is certainly inconvenient, you may be happy to know that you will likely not need to buy a brand-new gas water heater! One of the most common reasons for water heater problems is a bad thermocouple. Now, you are probably wondering, what does a thermocouple do on a water heater?

Good question! Read on for the answer!

thermocouple in heater

So, what does a thermocouple do on a water heater? A thermocouple acts as a safety device that tells your water heater if you have a working pilot light. If the pilot light is not working, the thermocouple will not open the gas valve, which keeps any unburned natural gas from entering your house.

Do you want to learn more about faulty thermocouples and how to test for them? Keep scrolling down for everything you need to know!

Does My Electric Water Heater Use a Thermocouple?

No, it does not. Electric water heaters do not use gas, so they do not need a thermocouple to act as an emergency gas shutoff.

Are Thermocouples and Flame Sensors the Same Thing?

While the terms may be used interchangeably, no, they are not the same thing. Thermocouples are used on standing gas-powered appliances with pilot lights and check that the pilot lights are working properly. A flame sensor is used on appliances with electric ignitions, and they work by igniting the burners once they start to release gas.

How Do You Know if You Have a Bad Thermocouple?

The first sign that you may have a bad thermocouple is that you cannot use heat or hot water because your furnace or water heater keeps going off. This indicates that something is wrong with your pilot light, and the thermocouple is part of the pilot light burner assembly. While it is possible that there is another reason for your pilot light issues, it could very well be a bad thermocouple.

Will a Pilot Light Stay Lit with a Bad Thermocouple?

No, it will not. The water heater pilot light will ignite with a bad water heater thermocouple, but it will not stay lit. To light a pilot, you first light the pilot flame and then hold it in the main gas valve for 20-30 seconds to allow time for the thermocouple to get hot. If the pilot light goes out after you let go of the gas control valve, then there is likely something wrong with your thermocouple.

How Do You Test a Thermocouple on a Water Heater?

digital multimeter flat design concept electrical

To inspect and test your water heater thermocouple (this will work for furnaces, too!), try the following steps.

Step 1: Gather Materials

  • Wrench
  • Multimeter
  • Flame source (pilot light, match, candle, lighter, etc.)

Step 2: Inspect the Thermocouple

You will want to check the tube, wiring, and connectors. You could have a dirty thermocouple.

  • Check the tube for signs of contamination, such as pinholes, cracks, or discoloration.
  • Inspect the wiring for corrosion and damage, like a bare wire or missing insulation.
  • Look at the connectors for damage. This is important because damaged connectors may yield inaccurate results of the open-circuit test.

If anything needs to be cleaned, clean it. If you clearly have a damaged thermocouple, you can skip the next steps and order a replacement. Otherwise, if everything checks out, proceed to the next step.

Step 3: Turn Off the Gas and Remove the Thermocouple

Turn off the gas supply. Detach the thermocouple from the main gas valve by unscrewing the connection nut and copper lead, followed by the bracket nuts.

Step 4: Test and Set Your Multimeter

To test your multimeter, first set it to ohms. Then touch the two leads from the meter and ensure the meter reads zero. Then set the meter back to volts to perform the thermocouple test.

Step 5: Place Tip of Thermocouple Into Flame Source

Ignite your flame and place the tip of the thermocouple in it, allowing it to get hot.

Step 6: Attach the Multimeter to the Thermocouple

Put one lead from the multimeter on the end of the thermocouple in the flame source and the other lead on the thermocouple side.

Step 7: Understanding Your Multimeter Reading

If the thermocouple is working properly, the multimeter should read between 25 and 30 millimeters. If it is less than 25, it is time for a replacement thermocouple.

Can You Bypass a Thermocouple on a Water Heater?

thermocouple isolated on white background

You can, but it is not necessarily a good idea. A thermocouple is kind of like a safety switch in that it prevents unburned natural gas from getting into your house when your pilot light goes out.

If you have no way of telling when the pilot light goes out, there could be natural gas freely flowing into your home, which puts you at risk for natural gas sickness. The symptoms of natural gas sickness include headaches, memory problems, nausea, fatigue, and even loss of consciousness, so it may not be worth the risk.

What Causes Thermocouple to Fail?

There are two main reasons why a thermocouple will fail over time: metal fatigue and oxidation.

Metal Fatigue

Thermocouples get exposed to a lot of temperature changes over their lifetime. This causes the metal to expand and contract depending on the temperature, which eventually causes the thermocouple to weaken, resulting in metal fatigue.


Oxygen exposure can react with the pure metal of the thermocouple, which are commonly made with highly conductive materials such as copper, nickel, and/or iron. The reaction between the metal and oxygen can wear down the wires in the faulty thermocouple, making them thin and brittle.

How Much Should a Plumber Charge to Replace a Thermocouple?

Generally, it will cost up to $150 for a professional plumber to come in and replace a thermocouple.

How Long Does a Thermocouple Last?

This really depends on how often you use your water heater and if you perform any routine maintenance. If it is not being used constantly, was properly installed, and has been well-maintained, your thermocouple could last up to 10 years.

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