How to Clean a Furnace Flame Sensor

A furnace flame sensor is a vital component that ensures your furnace is operating safely and efficiently. Over time, dust, dirt, and debris can accumulate on the flame sensor, affecting its ability to detect the flame. Cleaning your furnace flame sensor regularly helps avoid problems like the furnace not igniting or shutting off unexpectedly. With some simple cleaning steps, you can easily keep your furnace flame sensor in proper working order.

What is a Furnace Flame Sensor and What Does it Do?

The flame sensor, also known as a flame rod, is a metal rod located inside the burner chamber of your gas furnace. It works alongside the igniter to signal that a flame is present after the gas valve opens and the igniter sparks.

Here’s how it works:

  • When your thermostat calls for heat, it sends a signal to the furnace control board to start the ignition sequence.
  • The gas valve opens to allow fuel into the burner chamber and the igniter creates a spark to light the gas.
  • Once the gas ignites and flame is present, the flame sensor signals the control board by conducting electricity between itself and the burner frame.
  • If the control board does not receive this signal within the trial period (usually 30-90 seconds), it will stop the heating cycle and lock out the furnace as a safety precaution.
  • When flame is detected, the control board keeps the gas valve open so your furnace can continue heating.

So in summary, the flame sensor provides a crucial feedback loop to prove there is flame and prevent your furnace from running without it. Keeping it clean allows it to accurately detect the flame.

Signs Your Flame Sensor Needs Cleaning

Some clear signs that your furnace flame sensor needs cleaning include:

  • Furnace fails to ignite: If the flame sensor is covered in debris, it cannot detect flame. This will prevent the furnace from igniting as part of the safety lockout.
  • Furnace ignites but shuts down: The furnace may light briefly but shut off after a few minutes if the flame sensor cannot consistently sense the flame.
  • Repeated ignition cycles: The furnace may go through multiple cycles of igniting, failing to detect flame, and shutting down before finally staying lit. This results in the furnace constantly restarting its ignition sequence.
  • Soot buildup: Excess soot on the flame sensor indicates it’s not heating up enough from the flame, likely due to dirt interfering with flame detection.
  • Burning smell or high CO levels: Inability to detect flame properly can result in unburned gas, leading to a burning odor or high carbon monoxide levels.

If you notice any of these issues, cleaning the furnace flame sensor should be your first step in troubleshooting.

How Often Should You Clean the Flame Sensor?

Most experts recommend inspecting your furnace flame sensor annually and cleaning it as needed. However, some situations may require more frequent cleaning:

  • Furnaces near construction zones or exposed to a lot of dust may need cleaning quarterly.
  • Homes with pets or excessive dirt buildup may need cleaning twice a year.
  • If you notice symptoms like repeated restarting, inability to stay lit, or soot buildup, clean the sensor right away before attempting further diagnostics.
  • Older furnaces may require more cleaning than newer models. New flame sensors may need only annual cleaning, while sensors over 5-10 years old may need biannual attention.
  • Flame sensors on furnaces that run for longer cycles or at higher temperatures can experience faster debris buildup.

Use the signs above as indicators to clean your furnace flame sensor as needed, even if it’s sooner than your yearly maintenance. Catching dirty flame sensors early prevents frustrating furnace issues.

Furnace Flame Sensor Cleaning Steps

Cleaning your furnace flame sensor is a quick and straightforward DIY project. Follow these key steps:

Turn Off Power to the Furnace

! Before any furnace maintenance, shut off power to avoid electric shock. This includes shutting off the circuit breaker that supplies power to your furnace.

Remove the Flame Sensor

  • Refer to your owner’s manual for specifics on accessing the flame sensor on your model. You will need to open the burner compartment cover.
  • Locate the flame sensor next to a burner. It is a thin metal rod about 2-3 inches long.
  • Remove any retaining clips or screws securing the flame sensor. Carefully slide it out for cleaning.

Gently Clean With Steel Wool

  • Use fine 000 or 0000 grade steel wool to lightly polish away debris from the sensor tip. Be careful not to bend the rod.
  • Visually inspect for any cracks or damage on the insulated portion. A damaged sensor should be replaced.
  • Clean until you remove all dirt and restore a shiny metallic surface. Avoid using sandpaper or wire brushes which can damage the rod.

Clean with Electrical Contact Cleaner

  • Spray electrical contact cleaner on the flame sensor tip to remove any remaining gunk.
  • Allow to fully dry before reinstalling. The fast-drying cleaner helps ensure a clean connection to detect flame.

Check Burner and Heat Exchanger

  • While you have the burner compartment open, check the burner and heat exchanger for any debris or cracks.
  • Use a vacuum and soft brush to gently remove any dirt or dust buildup.

Replace Flame Sensor

  • Once fully cleaned and dried, replace the flame sensor in the original position.
  • Secure any retaining clips or screws. Ensure the sensor is not touching any other burner components.
  • Close the burner compartment cover before restoring power.

Helpful Cleaning Tips

Follow these tips for an easier cleaning experience:

  • Use a flashlight to better see the sensor and where it connects.
  • Turn off power before you open the unit to avoid igniter sparking.
  • Handle the flame sensor gently without bending it to avoid damage.
  • Check for cracks or deterioration and replace if the sensor is defective.
  • Insert a business card between sensor and burner when scrubbing to protect the burner.
  • Clean until all debris is removed and the tip is shiny.
  • Be very careful not to scrape or damage the white insulated portion.
  • Allow sensor to dry fully before reinstalling to prevent corrosion.

Alternative Cleaning Methods

While steel wool and contact cleaner are typically recommended, the following alternatives can also work in a pinch:

  • Fine sandpaper: Only use very fine (2000+ grit) sandpaper. Gently polish.
  • Scotch-BriteTM scrubber: Using the green general purpose pad, gently scrub the sensor tip.
  • Baking soda and water paste: Make a paste of 3 parts baking soda to 1 part water. Gently scrub with a toothbrush.
  • White vinegar: Spray white vinegar and use a toothbrush to scrub. Then wipe clean.
  • Toothbrush and water: For mild dirt, a toothbrush and water may suffice.

Avoid using anything too abrasive like rubbing alcohol, wire brushes, or steel wool above 000 grade which can scratch the rod. The best cleaners for flame sensors have light abrasives and evaporate quickly.

Can I Clean the Flame Sensor Without Shutting Off the Furnace?

The short answer is no. For safety, you should always turn off the power supply to the furnace before conducting maintenance or removing any components. Here are some key reasons to avoid cleaning a furnace flame sensor while powered on:

  • You risk igniter sparking and ignition while your hands are near the burner. This could cause serious burns or injury.
  • Even with the thermostat off, touching internal components can potentially trigger electrical hazards or shorts.
  • You are unable to fully access, inspect, and clean the sensor and burner area with the furnace able to power on.
  • Any accidental bending or damage to the sensor while powered risks damaging furnace components.
  • Troubleshooting will be ineffective since you cannot fully clean the sensor or diagnose other issues with furnace on.

While it may seem convenient not to power down, there are too many safety hazards. Take the right precautions of turning off the furnace at the breaker before attempting flame sensor cleaning.

Flame Sensor Checking Steps After Cleaning

After completing the cleaning process, use these steps to safely startup and validate your furnace is working correctly:

  • Restore power to the furnace at the circuit breaker and turn on the thermostat.
  • Initiate a call for heat and observe furnace operation as it progresses through ignition.
  • Check that the igniter glows red to ignite the gas burners. Listen for a smooth ignition without multiple retry cycles.
  • Confirm the furnace stays lit and does not quickly drop out after ignition. Let it run 5+ minutes.
  • Monitor for any trouble codes on your control board display if equipped. Codes may persist until the board resets after a clean cycle.
  • Inspect the flame sensor area for any signs of soot buildup starting again indicating a still-dirty sensor.
  • Check for proper warmth coming from ductwork around 10-15 minutes after successful ignition.

If ignition fails completely, cycles repetitively, or the furnace fails to stay lit, you may need to clean the sensor further or replace it if defective. Contact a technician if cleaning attempts are unsuccessful.

When Should a Flame Sensor Be Replaced?

While cleaning the furnace flame sensor often restores normal operation, replace the sensor if you observe:

Physical damage – If the insulator or rod itself is cracked or chipped, replacement is required. Damage that exposes wiring can impact sensor function.

Corrosion – Heavy corrosion that cannot be scrubbed away indicates moisture damage. The sensor may need replacement.

Old age – Flame sensors have a typical lifespan around 5-10 years. If your sensor is outdated, replacing it can resolve issues.

Failed cleaning attempts – If thorough cleaning fails to resolve ignition or staying lit issues, the sensor likely needs replacement due to wear.

Repeated short cycling – Frequent short cycling points to a sensor unable to reliably detect flame, signaling replacement.

Replacement flame sensors can be purchased from HVAC supply stores for roughly $30-60. An HVAC technician may be able to replace it in under an hour during a service call.

FAQs About Cleaning Furnace Flame Sensors

How often should the furnace flame sensor be cleaned?

Inspect the flame sensor annually and clean as needed. Clean immediately if you notice ignition issues. Furnaces prone to heavier debris buildup may need cleaning every 6 months.

What happens if the flame sensor is bad?

If defective, the flame sensor will fail to detect a flame and lead to ignition failure, cycling, or furnace shutdown. Replacement is needed for a sensor that is damaged or fails cleaning attempts.

Where is the flame sensor located in a furnace?

The flame sensor is positioned within the burner compartment. It is a thin metal rod about 2-3 inches long next to a burner tube. Consult your model’s diagram to locate it.

Can a dirty flame sensor cause carbon monoxide?

Yes, an excessively dirty flame sensor can indirectly cause higher carbon monoxide levels by preventing proper flame detection. This may allow unburned gas to accumulate.

Why does my furnace keep turning on and off?

If your furnace cycles on and off repeatedly, a dirty flame sensor is a likely cause. The sensor cannot maintain the flame signal, leading to shutdowns. Cleaning it usually resolves this.

Does a flame sensor need AC or DC current to work?

Flame sensors operate by conducting microamps of DC current between the rod and burner chassis to prove the presence of flame. The system does not require external power.

What voltage should a furnace flame sensor read?

A properly conducting flame sensor will register between 0.5 – 1.5 microamps when flame is present. Out of range readings can indicate a dirty or defective sensor.

Can I jump a flame sensor?

No, bypassing the sensor defeats the crucial safety function it provides. With no flame verification, a furnace can continue operating while dangerously overheating.

How can I test my furnace flame sensor?

Turn off power and disconnect the sensor wires. Using a multimeter, you should measure continuity between the sensor tip and chassis ground with the sensor heated up. No continuity indicates a bad sensor.

Why does my furnace flame sensor need to be cleaned?

Dust, dirt, debris, and combustion byproducts gradually coat the sensor over time, insulating it from flame contact. Cleaning removes this buildup so the sensor can accurately detect the microamps induced by flame.


Regularly cleaning your furnace flame sensor is essential preventative maintenance to keep your furnace operating safely and efficiently all winter long. While cleaning the sensor is not complicated, remember to always power down your furnace at the breaker box first for your safety.

Carefully remove and clean the sensor using either steel wool, electrical contact cleaner, or gentle scrubbing. Visually inspect for damage and replace if the sensor is defective. Proper cleaning should allow your furnace to smoothly ignite and maintain the flame signal.

Catching dirty furnace flame sensors early by annual inspection prevents frustrating and costly issues like repeated cycling, failure to ignite, or unnecessary technician callouts. By following the steps outlined, you can easily clean your furnace flame sensor yourself and ensure your home stays warm all winter.