How to Test a Flame Sensor: A Comprehensive Guide (2024)

Flame sensors are critical components in gas-fired appliances, such as furnaces, water heaters, and boilers, responsible for detecting the presence of a flame and ensuring safe operation. Properly testing a flame sensor is essential to ensure the appliance is functioning correctly and to prevent potential safety hazards. In this comprehensive guide, we will delve into the step-by-step process of testing a flame sensor, providing you with the technical details and expert insights to perform this task effectively.

Understanding Flame Sensor Operation

A flame sensor is a safety device that monitors the presence of a flame in a gas-fired appliance. It works by detecting the small electrical current (known as the “flame signal”) generated when the flame ionizes the gas. This current is typically in the range of 0.5 to 10 microamps (μA), with a normal operating range of 2 to 6 μA.

The flame sensor is positioned in the path of the flame, and when the flame is present, it generates a small electrical current that is detected by the appliance’s control board. If the flame sensor does not detect a flame, the control board will shut off the gas supply to the appliance, preventing the buildup of unburned gas, which could lead to a dangerous situation.

Preparing to Test the Flame Sensor

How to Test a Flame Sensor: A Comprehensive Guide (1)

Before testing the flame sensor, ensure that the appliance is turned off and the power is disconnected. This is a crucial safety step to prevent any potential electrical hazards. Additionally, make sure that the appliance has cooled down completely before proceeding with the test.

Testing the Flame Sensor

To test the flame sensor, you will need a multimeter capable of measuring microamps (μA). Follow these steps:

  1. Locate the Flame Sensor: Identify the flame sensor within the appliance. It is typically a metal rod or probe located near the burner.

  2. Disconnect the Flame Sensor: Carefully disconnect the wire or connector that is attached to the flame sensor.

  3. Set the Multimeter: Set your multimeter to the microamp (μA) scale, with a resolution of at least 0.10 μA.

  4. Connect the Multimeter: Connect one lead of the multimeter to the flame sensor wire or connector, and the other lead to the terminal on the appliance where the flame sensor was connected.

  5. Turn on the Appliance: Turn on the appliance and observe the flame sensor reading on the multimeter.

  6. Evaluate the Reading: The flame sensor should read between 0.5 and 10 μA, with a normal range of 2 to 6 μA. If the reading is outside this range, the flame sensor may need to be cleaned or replaced.

Cleaning the Flame Sensor

If the flame sensor is not working correctly, it may need to be cleaned. Follow these steps:

  1. Turn off the Appliance: Ensure that the appliance is turned off and the power is disconnected.

  2. Remove the Flame Sensor: Carefully remove the flame sensor from the appliance.

  3. Clean the Flame Sensor: Gently clean the flame sensor rod or probe using a soft steel wool pad or a fine-grit sandpaper. Avoid using abrasive materials that could damage the sensor.

  4. Reinstall the Flame Sensor: Carefully reinstall the cleaned flame sensor back into the appliance.

  5. Retest the Flame Sensor: Repeat the testing process to ensure the flame sensor is now functioning within the normal range.

If cleaning the flame sensor does not improve the readings, the sensor may need to be replaced. In this case, it is recommended to consult a qualified technician to perform a thorough inspection of the appliance and replace the flame sensor if necessary.

Factors Affecting Flame Sensor Performance

Flame sensors can fail due to various reasons, including:

  • Cracked Insulators: Cracks in the insulation around the flame sensor can cause electrical leakage, leading to inaccurate readings.
  • Broken Sensors: The flame sensor rod or probe may become damaged over time, resulting in a malfunctioning sensor.
  • Improper Placement: If the flame sensor is not positioned correctly within the flame, it may not be able to detect the flame properly.
  • Buildup of Deposits: Accumulation of silica, carbon, or other deposits on the flame sensor can interfere with its ability to detect the flame.

It’s important to note that typical flame-sensing rods do not have a special coating that can be rubbed off with improper cleaning. Gentle cleaning with a soft steel wool pad or fine-grit sandpaper is generally recommended to maintain the sensor’s performance.

Ensuring Proper Grounding and Polarity

When testing a flame sensor, it is crucial to ensure that the appliance is properly grounded and that the polarity is correct. Improper grounding or reversed polarity can lead to inaccurate readings or even damage the flame sensor or the appliance’s control board.

Make sure that the appliance is securely grounded and that the multimeter leads are connected in the correct polarity, with one lead connected to the flame sensor wire and the other lead connected to the terminal on the appliance where the flame sensor was connected.


Testing a flame sensor is a critical task in maintaining the safe and efficient operation of gas-fired appliances. By following the step-by-step process outlined in this guide, you can effectively test the flame sensor, identify any issues, and take the necessary steps to ensure the appliance is functioning correctly. Remember to always prioritize safety and, if in doubt, consult a qualified technician for assistance.

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How to Test a Flame Sensor: A Comprehensive Guide (2024)


How many ohms should a flame sensor read? ›

With a good flame and a good Cad Cell, the resistance reading should usually be between 300 to 1000 ohms. The oil primary control is usually looking for an electrical resistance below 1600 ohms in order to verify the flame is present.

What is a good flame sensor reading? ›

When the burners ignite you should read between . 5 and 10 micro-amps depending of the furnace. Readings between 2 - 6 are most common. However, check the manufacturer's specs.

What voltage should a flame sensor read? ›

Now we want to check flame sensor voltage.

With your voltmeter, check the voltage between the sensor wire and the burner - it should be 90-107 volts AC.

Can you jump out a flame sensor? ›

Now that you know you can't bypass a flame sensor and need to buy a new one, shop around PlumbersStock for the right replacement flame sensor.

How many amps is a flame sensor? ›

When the flame lights, you should read between 0.5 and 10 microamps (μA), depending on the furnace. Readings between 2 and 6 are common.

How do you reset a flame sensor? ›

Turning the power off for approximately 20 seconds and turning it back on again will reset most modern gas furnaces. Some furnace models can power on again after leaving the lockout for 1-2 hours. The cleanliness of the flame sensor of a furnace is very important.

What to do if flame sensor is not working? ›

Start by turning off the power to the furnace and locating the flame sensor, usually a metal rod near the burner assembly. Gently clean it using a soft cloth or fine-grit sandpaper to remove any buildup or residue. After cleaning, reinstall the sensor and turn on the power to see if it resolves the issue.

Can you touch a flame sensor? ›

Do not touch the flame sensor with your fingers. The oils on your hands can get on the sensor and make it more prone to get dirty later.

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