When Should You Replace Your Furnace Flame Sensor?

Does your furnace light normally and then shut off again after only a few seconds?

A dirty, damaged or poorly performing flame sensor may be the cause. The furnace may attempt ignition several times before entering a safety lockout and stopping the heating cycle.

Many online guides recommend cleaning the flame sensor. Cleaning can often restore normal operation, but it is not always a permanent solution.

The important question is not simply whether the sensor can be cleaned. It is whether the sensor is still producing a reliable flame signal and whether another furnace problem is contributing to the shutdown.

What Does a Furnace Flame Sensor Do?

A furnace flame sensor is an important safety device that confirms the burners have ignited successfully.

When the thermostat calls for heat, the furnace begins its ignition sequence. After the gas valve opens and the burners ignite, the flame sensor detects the burner flame and sends a small electrical signal to the furnace control board.

As long as the control board receives an adequate flame signal, it keeps the gas valve open and allows the furnace to continue heating.

If the control board does not detect a proper flame signal, it closes the gas valve within a few seconds. This helps prevent unburned gas from continuing to enter the furnace.

Signs of a Flame Sensor Problem

Possible signs of a dirty, weak or failing flame sensor include:

• The burners ignite and shut off after a few seconds
• The furnace repeats the ignition sequence several times
• The furnace enters an ignition lockout
• The furnace works again after the flame sensor is cleaned
• The same shutdown problem keeps returning
• The flame sensor rod is cracked, damaged or heavily corroded
• The electrical terminal or porcelain insulator is damaged

These symptoms do not automatically prove that the flame sensor is defective. Similar problems may be caused by poor grounding, damaged wiring, burner contamination, improper flame contact or a furnace control issue.

Why Do Flame Sensors Become Dirty?

During normal operation, a flame sensor is continually exposed to heat and combustion gases.

Over time, deposits may form on the metal sensing rod, including:

• Oxidation
• Carbon deposits
• Dust
• Rust or corrosion
• Other combustion by-products

These deposits can interfere with the sensor’s ability to produce a strong and reliable flame signal.

A sensor may look clean but still have a weak signal. A qualified HVAC technician can measure the flame-sensing current and compare it with the furnace manufacturer’s specifications.

Should You Clean or Replace the Flame Sensor?

Cleaning a flame sensor is commonly performed during furnace maintenance and may be all that is required when the sensor is undamaged and the flame signal returns to an acceptable level.

Replacement should be considered when:

• The sensor is physically damaged
• The porcelain insulator is cracked
• The electrical terminal is loose or corroded
• The sensing rod is badly pitted or deteriorated
• The flame signal remains weak after proper cleaning
• The problem repeatedly returns
• The manufacturer recommends replacement
• Testing confirms that the sensor is no longer operating reliably

Replacing a flame sensor without testing may not solve the problem if the actual cause is poor grounding, damaged wiring, dirty burners or improper burner flame.

Why Does the Furnace Shut Down?

The furnace control board must detect flame shortly after ignition.

If the flame signal is too weak or disappears, the control board closes the gas valve as a safety precaution. The furnace may then attempt ignition again.

After several unsuccessful attempts, many furnaces enter a safety lockout. Depending on the model, the furnace may remain off until the power is reset or until a timed lockout period has passed.

The furnace diagnostic light may also display an error code related to flame loss, ignition failure or flame not proven.

Do Not Ignore Repeated Ignition Problems

A furnace that repeatedly lights and shuts down should be inspected.

Repeated ignition attempts can leave your home without dependable heat and may indicate a problem beyond a dirty flame sensor.

Possible causes include:

• Dirty or misaligned burners
• Weak burner flame
• Poor furnace grounding
• Damaged flame-sensor wiring
• Incorrect gas pressure
• Improper combustion
• A faulty furnace control board
• A damaged flame sensor

A flame sensor is a safety component. Do not bypass it or attempt to operate the furnace without it connected.

Finding the Correct Replacement Flame Sensor

Flame sensors may look similar but differ in rod length, angle, mounting bracket, terminal connection and equipment compatibility.

To identify the correct replacement, provide:

• Furnace manufacturer
• Complete furnace model number
• Serial number
• Original flame-sensor part number
• A clear photograph of the existing sensor

Whenever possible, use the manufacturer’s specified OEM flame sensor for your furnace.

Order a Genuine OEM Furnace Flame Sensor

OEMHVACPartsCanada.ca supplies genuine OEM flame sensors for many leading furnace manufacturers, with shipping throughout Canada.

Whether you are an HVAC technician or a qualified homeowner searching for a replacement, we can help identify the correct flame sensor using your furnace model number and existing part information.

Browse our available furnace flame sensors or contact us for assistance identifying the correct OEM replacement part.

When Should You Call an HVAC Technician?

Contact a qualified HVAC technician when:

• You smell natural gas or propane
• The furnace repeatedly enters lockout
• The burners do not ignite properly
• The flame appears abnormal
• The problem returns after cleaning
• Electrical testing or combustion testing is required
• You are not trained to work safely on gas-fired equipment

Turning off electrical power does not eliminate every hazard associated with a gas furnace. Diagnosis and repairs should be completed by a qualified person when required.