Water Heater Won’t Light: How to Test and Replace a Faulty Thermocouple

Water Heater Won’t Light: How to Test and Replace a Faulty Thermocouple

A water heater is one of the best appliances in any household or business establishment, as the device supplies heated water for cleaning, cooking, and daily comfort. When the water heater fails to ignite, it tends to be a problem with the thermocouple—a small and vital safety device. Understanding how to do a water heater thermocouple test, what conditions warrant an attempt at cleaning a water heater thermocouple, and what conditions necessitate a water heater thermocouple replacement can spare a person both time and money.

This article will take you through the basics of what a thermocouple is, the indicators of its possible failures, the testing of the hardware, and its safe replacement when the situation arises.

Common Signs of a Faulty Thermocouple and Their Fixes

When your water heater does not want to remain lit, you can blame a damaged thermocouple. It can be fixed by identifying its warning symptoms early enough and implementing the right remedies to restore safe, reliable performance.

Pilot Light Won’t Stay Lit

When a pilot light ignites but does not remain lit, it is one of the most concrete proofs of a poor thermocouple. With the thermocouple designed to sense the heat of a flame and produce a tiny amount of electricity to hold the gas valve open, failure cuts off the gas and extinguishes the flame.

Fix:

• Cleaning the thermocouple water heater transferred deposits of soot or carbon wash over the tip, which sometimes will block it. Sensitive tips can be restored by soft sanding the tip with fine-grit sandpaper.

• Check alignment. The thermocouple tip has to be placed in the pilot flame. Move it to such a distance where you can see it easily, but not too far.

• When cleaning and straightening do not suffice, a thermocouple replacement is probably needed in a water heater.

  
  

Weak, Flickering, or Yellow Pilot Flame

The pilot flame should be mild blue in color, and it should touch the tip of the thermocouple. When there is little or no flame or a yellow-looking flame, then either the thermocouple is not receiving sufficient heat or its wires are not securely attached. Although this may point to a clogged pilot tube, much depends on the location of the thermocouple.

Fix:

• Check the thermocouples' location to the flame. Modify it so the flame totally engulfs the tip.

• Clean the thermocouple and pilot assembly with clean brushes to remove the accumulation of dirt.

• If there has been no improvement in the strength of the flame, the pilot orifice will need to be cleaned or the T.C. changed.

  
  

Burner Won’t Ignite Even When Pilot Light Works

Other times the pilot flame remains lit, but the main burner does not ignite. This occurs when the thermocouple is not transmitting a signal to the gas valve that is powerful and gets noticed by the gas valve, despite the seeming proper functionality of the thermocouple.

Fix:

• Conduct a thermocouple test of a water heater with the use of a multimeter. A working thermocouple when connected to the circuit should give a reading between 25 and 35 millivolts. Anything less than 20 mV depicts weakness.

• In the case of a low reading, replacement of the thermocouple is the most suitable.

• Ensure that the connections made between the thermocouple and valve are firm. Loose Connections—Loose connections may resemble thermocouple failure

  
  

Visible Wear, Bends, or Corrosion on the Thermocouple

As with any other metal component subjected to prolonged direct heat, thermocouples wear out. Signs are a pitted and discolored or corroded tip or a visible bent rod that does not have a straight alignment with the pilot flame.

Fix:

• However, physical damage cannot be fixed by cleaning.

• The only solution here is how to change a water heater thermocouple by installing a new one.

  
  

Intermittent Hot Water Supply

If your water heater occasionally works and occasionally fails to, it may be an issue with a failing thermocouple. This typically is an indication of a bad or dirty thermocouple that is giving the gas valve erratic information.

Fix:

• Start by cleaning the thermocouple tip to get rid of any accumulated material.

• Ensure there is a very close connection between the thermocouple lead and the gas valve.

• In case of persistence, the long-term solution to the problem is replacement.

  
  

Delayed Ignition or Frequent Pilot Outages

However, in case you have noticed the frequent extinguishing of the pilot light or the long wait time of the water heater igniting during overnight hours, it could be an indication of the thermocouple life cycle at or near its end.

Fix:

• Test a thermocouple with a multimeter to read the output voltage.

• When the voltage is under the recommended levels, replace.

  
  

Steps to Test a Water Heater Thermocouple

If your water heater won’t stay lit, performing a water heater thermocouple test is the best way to confirm whether the component is still working. With the effect of the multimeter test, it can aid in reading the voltage output that is produced by the thermocouple and check whether it is giving the right signal as required by the gas valve. The following is a guideline:

Step 1: Gather the Necessary Tools

You’ll need:

• A digital or analog multimeter capable of measuring millivolts (mV)

• A wrench or pliers to disconnect the thermocouple

• Heat source (the pilot flame or a lighter)

• Safety gloves

  
  

Step 2: Shut Off the Gas Supply

The gas can be turned off by the shut-off valve before touching any component of the water heater. This creates a secure environment for testing and avoids accidental spills.

Step 3: Remove the Thermocouple

On the pilot light assembly point, the thermocouple is out. Unbolt it with a wrench off the gas control valve. Next, take it out slowly by easing it out of the bracket that keeps it in position.

Step 4: Connect the Multimeter

On your multimeter, set it to millivolts (DC). Insert the red probe in the copper terminal of the thermocouple and the black probe at the tips of the same thermocouple. This mechanism will enable you to get the voltage it produces when warmed.

Step 5: Heat the Thermocouple Tip

Place the thermocouple tip in the pilot flame or a lighter and heat for 30–40 seconds. As the metal warms, it should start producing some little current.

Step 6: Check the Voltage Reading

Note the multimeter reading. A good thermocouple would give 25–35 mV. When below 20 mV is the reading, the thermocouple is too weak to leave the gas valve open. No reading at all indicates a complete failure of the thermocouple.

Step 7: Reinstall or Replace

• Provided the reading is within the limit, rescrew the thermocouple back onto the gas valve, then reconnect firmly.

• If it shows weak or no output, it’s time for water heater thermocouple replacement.

  
  

How to Safely Replace a Faulty Thermocouple

If your water heater thermocouple test confirms low or no voltage, cleaning won’t help—you’ll need a water heater thermocouple replacement. Fortunately, it is not a complex procedure that requires exceptional equipment and skills to accomplish.

Step 1: Turn Off Gas and Power

Get the shutoff valve of the gas and turn it off. Unplug the power supply of your heater too, in case your heater has some electrical parts. Leave the gas a few minutes to be sure that there is no gas left.

Step 2: Remove Access Panels

A majority of the gas water heaters have lower panels that cover the burner and pilot assembly. To access the thermocouple, remove these panels.

Step 3: Disconnect the Old Thermocouple

Unscrew the thermocouple by the help of a wrench at the gas control valve. Slippery-Slide it out of the bracket next to the pilot light. Note the location of it, because the new thermocouple should be in the same place.

Step 4: Install the New Thermocouple

Insert the new thermocouple into the bracket in the correct orientation so that the end of the thermocouple is in the proper location where the pilot flame burns. Fit the other end into the gas valve and screw it on with a wrench—but not too tightly.

Step 5: Reassemble and Relight

Once the burner has been reassembled, all panels should be fixed into place. Turn on the gas again and repeat the steps in relighting the pilot light in your water heater. Test the flame to make sure that it does not go out and that the burner does not light abnormally.

A thermocouple has a tendency to malfunction when your water heater refuses to light. Acquiring an understanding of the failure symptoms, a water heater thermocouple test, water heater thermocouple cleaning, and water heater thermocouple replacement will show you how to resolve your water heating issue promptly and safely. To many homeowners, the ability to replace the thermocouple on their water heaters comes in handy, as it saves money and unscheduled repairs as well as time wastage.

Nevertheless, in case the problem continues even after replacing the part, or you suspect gas leaks, do not be reluctant to call on an expert. When maintained and repaired in time, your water heater can last years, providing you with hot water.

1. Can I replace a water heater thermocouple myself?

Yes, replacing a water heater thermocouple is a common DIY project, particularly for individuals with basic handyman skills.

2. How to test for a faulty thermocouple?

A defective thermocouple is frequently indicated by a pilot light that will not stay lit. A thermocouple can also be examined with a multimeter to see if it produces a low millivolt signal when heated, which indicates a problem.

3. What makes a thermocouple go bad?

Thermocouples can fail due to a variety of reasons, including mechanical damage, overheating, contamination, and age. These factors can lead to inaccurate readings or complete failure of the thermocouple. 

PartsHnC, offers HVAC replacement parts for air conditioners, water heaters, heat pumps, and ventilation systems—delivers quickly to ensure your equipment performs at its best.