How to Test Furnace Flame Sensor

How to Test Furnace Flame Sensor

To test a furnace flame sensor, first, turn off the power to the furnace, then remove the flame sensor. Using a multimeter, set it to the microamp reading mode and connect the multimeter leads to the flame sensor.

A good flame sensor will typically read between 3-6 microamps. If the reading is lower or zero, the flame sensor may be faulty. If the reading is within the normal range and the furnace still doesn’t work properly, further troubleshooting may be needed.

Understanding Your Furnace Flame Sensor

Understanding Your Furnace Flame Sensor is crucial for maintaining the efficiency of your furnace. Testing the flame sensor is a simple process that involves disconnecting the wire attached to it and using a meter to measure microamps. By following these steps, you can ensure that your furnace is running smoothly and avoid any potential issues.

Brief On The Furnace Flame Sensor Function

The furnace flame sensor is a crucial component of your furnace’s ignition system. It is responsible for detecting the presence of a flame and ensuring that it is burning properly. The sensor works by sending a small electrical current to the furnace’s control board, indicating that the flame is present.

By understanding how the furnace flame sensor functions, homeowners can better troubleshoot and maintain their furnace systems. The flame sensor is typically made of a metal rod that protrudes into the flame. When the furnace ignites, the sensor detects the heat and generates an electrical signal. This signal is then sent to the control board, which determines whether the furnace should continue operating or shut down for safety reasons.

Without a properly functioning flame sensor, the furnace may not ignite or may shut down unexpectedly. This can lead to discomfort for homeowners and potential damage to the furnace system if left unresolved.

Importance In The Ignition Process

The furnace flame sensor plays a crucial role in the ignition process. It ensures that the furnace ignites and produces heat efficiently and safely. When the thermostat signals the furnace to turn on, the gas valve opens, and the igniter starts to produce a spark or heat. The flame sensor is responsible for detecting whether the igniter has successfully ignited the gas.

If the flame sensor does not detect a flame, it sends a signal to the control board to shut down the furnace as a safety precaution. This prevents the buildup of gas in the furnace and reduces the risk of a gas leak or a potential fire hazard.

Testing Your Furnace Flame Sensor

Regularly testing your furnace flame sensor is essential to ensure its proper functioning. A malfunctioning flame sensor can result in a variety of issues, such as frequent furnace cycling, insufficient heat output, or even a complete system shutdown.

To test your furnace flame sensor, follow these steps:

  1. Turn off the power to your furnace.
  2. Locate the flame sensor, typically located near the furnace burners.
  3. Remove any dirt or debris from the sensor using a soft brush or cloth.
  4. Using a multimeter, set it to the microamps (µA) setting.
  5. Disconnect the wire attached to the flame sensor.
  6. Connect one lead of the multimeter to the flame sensor’s wire and the other lead to a ground source.
  7. Turn on the furnace power and set the thermostat to call for heat.
  8. Observe the multimeter reading. If it falls within the manufacturer’s specified range (typically 2-6µA), the flame sensor is functioning correctly. If the reading is outside this range, the sensor may need cleaning or replacement.

It’s important to note that if you are unsure how to perform the testing or feel uncomfortable doing so, it’s best to consult a professional HVAC technician. They have the knowledge and expertise to accurately diagnose and fix any issues with your furnace flame sensor.

Regularly testing and maintaining your furnace flame sensor is vital for efficient and safe furnace operation. By familiarizing yourself with its function and implementing proper testing procedures, you can ensure that your furnace ignites reliably and provides optimal warmth for your home.

Preliminary Safety Checks

Before testing the furnace flame sensor, it is crucial to perform some preliminary safety checks to ensure the safety of yourself and your home. These checks will help prevent any accidents or damages that could arise during the testing process.

Shutting Off Power To The Furnace

The first step in the preliminary safety checks is to shut off the power to the furnace. This is essential to avoid any electrical shocks or injuries while working on the furnace. Make sure to locate the circuit breaker that supplies power to the furnace and turn it off. If you are uncertain about which breaker controls the furnace, it is recommended to turn off the main breaker to ensure complete safety.

Tools Required For Testing

Testing the furnace flame sensor requires a few essential tools to accurately diagnose and troubleshoot any issues. Here are the tools you will need:

  • Multimeter: A multimeter is a versatile tool that allows you to measure various electrical readings, including microamps. This will help you determine if the flame sensor is functioning properly or needs cleaning or replacement.
  • Screwdriver: A screwdriver is needed to remove the furnace door and access the flame sensor. Make sure you have the appropriate type and size of screwdriver to avoid any damage to the furnace.
  • Wire brush: A wire brush is necessary to clean the flame sensor if it is covered in dirt or soot. This will help improve its performance and ensure accurate readings.
  • Protective gloves and goggles: Safety should always be a priority when working with any heating or electrical components. Wearing protective gloves and goggles will protect your hands and eyes from any potential hazards.

By performing these preliminary safety checks and having the necessary tools ready, you’ll be well-prepared to test the furnace flame sensor and ensure the efficient functioning of your heating system. Remember, safety should always come first when dealing with any electrical components, so take the necessary precautions to protect yourself and your home.

Locating And Accessing The Sensor

To test a furnace flame sensor, start by removing the door to the furnace and disconnecting the wire attached to the sensor. Adjust the thermostat and set your meter to measure microamps. If the flame sensor is out of range, it may need to be replaced.

Steps To Locate The Sensor

Locating the furnace flame sensor is the first step in testing it for proper functionality. Follow these steps:

  1. Turn off the power: Start by turning off the power to the furnace. This is crucial for your safety as well as preventing any damage to the furnace.
  2. Find the furnace access panel: Locate the access panel on the furnace. This panel is typically located on the front or side of the unit. Consult your furnace’s manual if you are unsure.
  3. Remove the access panel: Use a screwdriver or the appropriate tool to remove the screws or fasteners securing the access panel. Carefully remove the panel and set it aside.
  4. Locate the flame sensor: Once the access panel is removed, you should be able to see the internal components of the furnace. Look for a small metal rod protruding into the combustion chamber. This is the flame sensor.

How To Safely Access The Sensor

Now that you have located the flame sensor, it is essential to access it safely for testing. Follow these steps:

  1. Ensure the power is off: Before accessing the flame sensor, double-check that the power to the furnace is turned off. This step is crucial to avoid any accidents or electrical hazards.
  2. Disconnect the wire: The flame sensor is connected to the furnace’s control board by a wire. Use a screwdriver or pliers to carefully detach the wire from the sensor. Be gentle to avoid any damage to the wire or sensor.
  3. Inspect for dirt or corrosion: Take a close look at the flame sensor. Check for any dirt, debris, or corrosion that may be affecting its performance. If you notice any build-up on the sensor, it will need to be cleaned before testing.
  4. Clean the flame sensor (if needed): If the flame sensor is dirty or corroded, it is recommended to clean it before testing. Use a soft brush or emery cloth to gently clean the sensor. Be careful not to apply too much pressure or damage the delicate parts.
  5. Reconnect the wire: Once you have cleaned the flame sensor (if necessary), reconnect the wire to its original position. Ensure it is securely attached to the sensor and the control board.

These steps should help you locate and safely access the furnace flame sensor for testing. In the next section, we will discuss the process of testing the sensor to ensure it is functioning correctly. Stay tuned!

How to Test Furnace Flame Sensor


Inspecting For Visible Issues

Inspecting for Visible Issues: Learn how to test your furnace’s flame sensor with a step-by-step guide. By following these instructions, you can ensure your furnace is working efficiently and identify any potential problems.

Identifying Soot, Corrosion, Or Damage

Inspecting the flame sensor for visible issues is an essential step in maintaining the proper functioning of your furnace. During this inspection, you need to look for signs of soot, corrosion, or physical damage that could affect the sensor’s performance. These issues can prevent the sensor from accurately detecting the flame, leading to problems with the furnace’s operation.

When To Simply Clean Or Replace

Based on your visual inspection, you may find that the flame sensor only requires cleaning to restore its functionality. Soot and debris can accumulate on the sensor over time, causing it to misread the flame signal. To clean the sensor, carefully remove it from the furnace and gently scrub off any buildup using a soft brush or sandpaper. After cleaning, reinstall the sensor and test the furnace to see if the issue has been resolved.

However, if you notice signs of corrosion or physical damage, it is likely that the flame sensor will need to be replaced. Corrosion can impair the sensor’s ability to accurately detect the flame, while physical damage may compromise its overall effectiveness. Consider consulting a professional HVAC technician for guidance on selecting and installing a suitable replacement sensor.

Regular maintenance and inspection of the flame sensor can help ensure that your furnace operates efficiently and reliably. By addressing any visible issues promptly, you can prevent more significant problems from occurring and extend the lifespan of your furnace.

Step-by-step Sensor Testing

Learn how to test your furnace flame sensor step by step in this comprehensive guide. Avoid common errors and ensure accuracy with these easy-to-follow instructions. Keep your furnace running efficiently by regularly testing and maintaining the flame sensor.

Safety Precautions Before Starting

Before testing the furnace flame sensor, it is important to take certain safety precautions to ensure your well-being. These precautions include:
  1. Turn off the power: Locate the circuit breaker or switch that controls the furnace and switch it off to prevent any electricity-related accidents.
  2. Handle with care: The furnace flame sensor is a delicate component, so make sure to handle it with care to avoid any damage.
  3. Wear protective gear: Wear safety gloves and goggles to protect yourself from any potential hazards.
  4. Wait for cooling: Allow the furnace to cool down completely before attempting any testing to prevent burns or injuries.

Connecting The Multimeter

Once you have taken the necessary safety precautions, you can proceed with testing the furnace flame sensor using a multimeter. Follow these steps to connect the multimeter correctly:
  1. Locate the flame sensor: The flame sensor is typically located near the burner assembly in the furnace.
  2. Disconnect the wire: Carefully remove the wire attached to the flame sensor by gently pulling it off. Make sure to remember its placement for reconnection later.
  3. Set the multimeter: Turn on the multimeter and set it to measure microamps (µA). This setting will allow you to accurately test the flame sensor’s current.
  4. Connect the leads: Connect the black (negative) lead of the multimeter to the grounded metal part of the furnace, and the red (positive) lead to the flame sensor’s wire that you disconnected earlier.

Reading And Interpreting Results

With the multimeter successfully connected, you can now proceed to read and interpret the results. Follow these steps:
  1. Observe the reading: After turning on the furnace, observe the reading on the multimeter. A working flame sensor should generate a reading in the range of 2-4 microamps.
  2. Interpret the results: If the reading falls within this range, your flame sensor is functioning properly. However, if the reading is significantly lower or higher, it indicates a potential issue with the flame sensor.
  3. Clean or replace: If the reading is low or absent, the flame sensor may be dirty or faulty. In such cases, it is recommended to clean the sensor using a soft cloth or replace it entirely to restore proper functionality.
Remember, accurate results and interpretation are crucial for effective troubleshooting, maintenance, and repair of your furnace flame sensor. Make sure to follow the proper steps and consult a professional if needed.

Cleaning The Flame Sensor

To test your furnace flame sensor, start by removing the door to the furnace and disconnecting the wire attached to the sensor. Adjust the thermostat and set your meter to measure microamps to check if the sensor is within range.

If not, consider cleaning or replacing the sensor.

Cleaning Methods And Materials

When it comes to cleaning the flame sensor of your furnace, there are a few methods and materials you can use to ensure its proper functioning. Here are some effective cleaning techniques:

  1. Visual Inspection: Start by visually inspecting the flame sensor for any visible dirt, debris, or corrosion. Use a flashlight to get a clear view of the sensor.
  2. Compressed Air: If the sensor appears to have only minor dust or debris buildup, you can use compressed air to clean it. Ensure that the air pressure is not too high as it may damage the sensor.
  3. Brushing: Another effective method is to gently brush off any dirt or corrosion buildup on the sensor. Use a soft-bristled brush to avoid scratching the surface. Be sure to brush in the direction of the sensor’s length to prevent damage.
  4. Isopropyl Alcohol: For more stubborn buildup or corrosion, you can use isopropyl alcohol to clean the flame sensor. Dampen a soft cloth or cotton swab with the alcohol and gently wipe the sensor until it is clean. Avoid using excessive alcohol to prevent damage.

Remember to always follow the manufacturer’s instructions and turn off the power to the furnace before cleaning the flame sensor to ensure safety.

Reinstalling The Sensor

Once you have cleaned the flame sensor, it is important to reinstall it correctly to maintain its effectiveness. Follow these steps for proper reinstallation:

  1. Align the Sensor: Ensure that the flame sensor is properly aligned in its designated position. Refer to your furnace’s manual or the manufacturer’s instructions for the correct alignment.
  2. Tighten the Screws: Use the appropriate screwdriver to tighten the screws that hold the flame sensor in place. Make sure the sensor is securely fastened.
  3. Reconnect the Wire: Reconnect the wire to the flame sensor. Ensure the connection is tight and secure to maintain optimal functionality.
  4. Reset the Power: After reinstalling the sensor, turn on the power to the furnace and test its operation. Check for any error codes or issues that may indicate a faulty flame sensor.

By properly cleaning and reinstalling the flame sensor, you can ensure its accuracy and efficiency in detecting the flame and maintaining the safety and performance of your furnace.

Reactivating Your Furnace

To test your furnace flame sensor, start by removing the door to the furnace and disconnecting the wire attached to the sensor. Adjust the thermostat and set your meter to measure microamps. If the reading is out of range, you may need to replace the flame sensor.

Powering Your Furnace Back On

Once you have successfully cleaned the flame sensor on your furnace, it’s time to reactivate the unit. Follow these steps to power your furnace back on:

  1. Replace the furnace door: Carefully place the door back in its original position and make sure it is securely closed. This is important to ensure proper functioning of the unit.
  2. Restore power to the furnace: Locate the power switch for your furnace. In most cases, it is located near the unit or on the electrical panel. Flip the switch on to supply power to the furnace.
  3. Adjust the thermostat: Set the temperature on your thermostat to a level higher than the current room temperature. This will trigger the heating cycle and the furnace should start running.

Observing The Ignition Sequence

Now that your furnace is powered back on, it’s important to observe the ignition sequence to ensure the flame sensor is working properly. Follow these steps:

  1. Listen for the sound of the ignition: As the furnace starts up, you should hear a clicking sound followed by the sound of the gas igniting. This indicates that the ignition sequence has begun.
  2. Watch for the flame: Observe the burner assembly through the sight glass or access panel. A blue flame should appear and continue to burn steadily. This confirms that the flame sensor is functioning correctly.
  3. Check for any error codes: Some furnaces have built-in diagnostic systems that display error codes in case of any issues. Refer to your furnace’s user manual to interpret these codes, if applicable.

What To Do If The Sensor Fails Post-cleaning

If you have followed the steps to clean the flame sensor and power your furnace back on, but the sensor fails to work properly, don’t panic. Here are a few things you can do:

  1. Inspect for loose connections: Ensure that the wire connected to the flame sensor is securely attached. If it appears loose or damaged, try tightening the connection or replacing the wire.
  2. Check for debris or residue: Double-check that there is no remaining debris or residue on the flame sensor. Use a soft brush or cloth to gently clean the sensor again, if needed.
  3. Consult a professional: If you have followed all the steps and the flame sensor still fails to function correctly, it may be time to call a professional HVAC technician. They will have the expertise to further troubleshoot and resolve the issue.

By following these steps to reactivate your furnace and observing the ignition sequence, you can ensure that your furnace flame sensor is working optimally. If you encounter any issues, don’t hesitate to seek professional assistance for a thorough diagnosis and resolution.

Frequently Asked Questions Of How To Test Furnace Flame Sensor

How Do I Know If My Furnace Flame Sensor Is Bad?

If you suspect a bad furnace flame sensor, look for these signs: 1. Furnace turns on but shuts off after a few seconds. 2. Cracks on the porcelain protecting the sensor. 3. Soot and debris covering the sensor. 4. Corrosion on the sensor.

5. Black tip on the sensor.

Can You Test A Flame Sensor With A Multimeter?

Yes, you can test a flame sensor with a multimeter. Turn off the power, remove the sensor, and use the multimeter probes to measure the sensor’s resistance. If the resistance is outside the normal range, the sensor may be faulty.

How Many Ohms Should A Flame Sensor Read?

A flame sensor should read between 1. 5 to 4. 8 ohms to function properly.

Is There A Way To Bypass A Flame Sensor On A Furnace?

To bypass a flame sensor on a furnace, it is not recommended as it poses a safety risk. It is best to troubleshoot and repair the issue with the flame sensor to ensure proper functioning of the furnace.


To conclude, testing your furnace flame sensor is a crucial step in ensuring the efficiency and functionality of your heating system. By following the step-by-step guide outlined in this blog post, you can easily diagnose and address any issues with your flame sensor.

Remember to use a multimeter to measure microamps and check for any signs of damage or corrosion. By regularly testing and maintaining your flame sensor, you can prevent potential breakdowns and ensure optimal performance for your furnace.

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