How to Tell if a Furnace Flame Sensor is Bad

Ensuring your home remains warm and safe during the colder months largely depends on the efficient operation of your furnace. Central to this efficiency is a small, but crucial component known as the flame sensor. This sensor plays a pivotal role in your furnace’s safety mechanisms, monitoring the presence of a flame and preventing potential hazards. However, like any component, it can fail over time, leading to furnace malfunctions. Understanding the signs of a failing flame sensor, how to clean it, and when replacement is necessary is essential for maintaining your furnace’s performance and your home’s safety.

What is a Flame Sensor?

A flame sensor is a safety device located within the burner assembly of your furnace. As mentioned, it plays a pivotal role in monitoring the presence of a flame when your furnace is operating. If the sensor does not detect a flame, it will shut down the furnace to prevent unburned gas from accumulating and creating a dangerous situation inside your home. This precaution helps avoid the risks of gas leaks or explosions, protecting your household from potential carbon monoxide poisoning and fire hazards​​.

Flame Sensor vs. Thermocouple: Understanding the Difference

While both the flame sensor and thermocouple are crucial safety features within a furnace, they serve slightly different purposes and are found in different types of furnaces.

• Thermocouple: Typically found in older gas furnace models that use a standing pilot light. A thermocouple works by keeping the pilot light ignited, ensuring the gas valve remains open when a flame is present. It consists of a small flame that burns continuously, heating the tip of the thermocouple. If the flame extinguishes, causing the tip to cool, the thermocouple responds by shutting off the gas valve to prevent gas flow, thereby avoiding a potential safety hazard.
• Flame Sensor: Used in modern gas furnaces that operate with an electronic ignition system instead of a standing pilot light. Unlike the thermocouple, flame sensors are part of furnaces that utilize electronic igniters to light the gas. The sensor’s job is to confirm that the burners have ignited the gas successfully. Should the sensor detect a failure in the burner, it immediately turns off the gas, preventing unburned gas from entering the home.

The key distinction lies in their application: the thermocouple is a fixture in older furnaces with a continuous pilot light, while the flame sensor is a feature of newer, electronically ignited furnace models. This evolution reflects advancements in furnace technology, focusing on increased safety and energy efficiency​​.

8 Signs of a Malfunctioning Flame Sensor

Recognizing the signs of a malfunctioning flame sensor can help you address problems early, avoiding potential safety risks and costly repairs. Here are common indicators that suggest your furnace’s flame sensor may be bad:

1. Furnace Burners Turn On and Then Go Out After a Few Seconds

One of the most telltale signs of a faulty flame sensor is when the furnace burners ignite but fail to stay lit for more than a few seconds. This phenomenon, often referred to as “short cycling,” occurs because the sensor cannot detect the flame properly, prompting the safety mechanism to shut down the burners to prevent gas accumulation​​​​.

2. Yellow Burner Flames

A healthy furnace flame is typically blue, indicating complete combustion. If you notice yellow flames, it signifies incomplete combustion, which can result from a malfunctioning flame sensor not properly monitoring the flame. Yellow flames can lead to dangerous carbon monoxide levels in your home​​.

3. Rusted Flue Lines

Rusted flue lines may indicate poor venting, often caused by a faulty flame sensor. The flue system expels hazardous gasses outside your home. When compromised, it poses significant safety risks, including the potential for carbon monoxide poisoning​​.

4. High Utility Bills

An unexpectedly high heating bill can be a symptom of an inefficiently operating furnace. If the flame sensor is malfunctioning, your furnace may struggle to maintain the desired temperature, leading to increased gas consumption and higher bills​​.

5. Old Age

Furnaces and their components wear down over time. If your furnace is over 20 years old, components like the flame sensor may not function as effectively, leading to intermittent furnace operation and other issues related to aging hardware​​.

6. Exhaust Smells

A malfunctioning flame sensor can lead to incomplete combustion, resulting in exhaust gasses lingering inside your home. These gasses can have a distinct smell and are harmful to your health​​.

7. Constant Thermostat Adjustments

Finding yourself constantly adjusting the thermostat to maintain comfort could indicate a malfunctioning flame sensor. When the sensor fails, it can lead to uneven heating and a noticeable decrease in indoor comfort levels​​.

8. Short Cycling

Apart from the burners turning off shortly after ignition, short cycling can manifest in frequent on-and-off cycles of the furnace. This not only reduces efficiency but also puts unnecessary stress on your furnace, potentially leading to other mechanical failures​​​​.

Step-by-Step Guide for Cleaning a Flame Sensor

Maintaining your furnace’s efficiency and safety involves regular cleaning of the flame sensor. A clean flame sensor ensures accurate detection of the flame, preventing unnecessary shutdowns of your furnace. Here’s a detailed walkthrough on how to safely clean the flame sensor:

Step 1: Turn Off the Power

Safety first. Before you start, ensure the furnace’s power is completely turned off. This can usually be done at the furnace switch or the main electrical panel by turning off the breaker assigned to the furnace. This step is crucial to prevent any electrical accidents.

Step 2: Locate the Flame Sensor

The flame sensor is typically found near the furnace burner assembly. It’s a small rod, often made of metal, positioned so that it can directly sense the burner’s flame. The sensor usually has one or two wires connected to it and is mounted with one or more screws. If you’re unsure of its location, refer to your furnace’s manual for guidance.

Step 3: Remove the Sensor

Using the appropriate screwdriver, carefully remove the screws that hold the flame sensor in place. Gently pull the sensor out of its mount, being mindful of the wires and connectors. If it’s the first time you’re doing this, take a photo before removal to help you remember how to reinstall it correctly.

Step 4: Clean the Sensor

Once removed, clean the flame sensor using a soft cloth or a fine abrasive pad. The goal is to remove any soot, dust, or corrosion without damaging the sensor itself. It’s important not to use anything too abrasive, as it could scratch or damage the sensor. Gently wipe or rub the rod until it’s clean and shiny again. Some sources recommend using steel wool or an emery cloth for this task, but always use the gentlest method possible to avoid damage​​​​.

Step 5: Reinstall the Sensor

After cleaning, carefully reinsert the flame sensor back into its mounting bracket. Secure it with the screws you removed earlier, ensuring it’s properly aligned as it was before removal. If you took a photo before starting, now is a good time to refer to it to ensure everything looks correct.

Step 6: Turn the Power Back On

Once the flame sensor is securely reinstalled, turn the power back on at the electrical panel or the furnace switch. It’s advisable to test the furnace after cleaning the sensor to ensure it operates correctly. Watch the furnace cycle to confirm that the burners ignite and stay lit, indicating a successful cleaning.

When to Replace a Flame Sensor

While regular cleaning can significantly extend the life and improve the performance of your furnace’s flame sensor, there are situations where cleaning alone might not suffice. Recognizing when a replacement is necessary can save you from the hassle of persistent furnace issues and ensure your home remains safe and warm. Here are key indicators that it might be time to replace your flame sensor:

Persistent Issues Despite Cleaning

If your furnace continues to experience the same issues after you’ve cleaned the flame sensor, such as failing to stay lit or shutting off unexpectedly, this could indicate that the sensor is beyond repair. Persistent problems often mean the sensor can no longer accurately detect the flame, even when it appears to be clean and properly positioned​​.

Severe Damage

Upon inspection, if the flame sensor shows signs of severe damage, such as cracks, significant corrosion, or other physical deformations, it’s a clear signal that cleaning will not restore its functionality. Damage to the sensor can impair its ability to conduct electricity and properly sense the presence of a flame, necessitating a replacement​​​​.

Inability to Rectify Burner Issues

Sometimes, burner problems persist even after the flame sensor has been cleaned. This can include the burners not lighting at all or lighting and then going out shortly after. If troubleshooting doesn’t resolve these issues, it could be due to a malfunctioning flame sensor that fails to communicate the presence of a flame to the furnace’s control board, signaling the need for a replacement​​​​.

Consider the Age of Your Furnace

The age of your furnace can also play a significant role in deciding whether to replace the flame sensor. If your furnace is old and nearing the end of its expected lifespan, replacing a faulty flame sensor might be a temporary fix to a system that needs a more comprehensive update or replacement. In such cases, consulting with a professional HVAC technician can provide insight into the most cost-effective and practical solution​​.

Keep Your Home Warm with Legacy Air’s Expert Furnace Care

The furnace flame sensor is crucial for your home’s safety, ensuring your heating system operates only when it’s safe. Recognizing malfunction signs and maintaining this component can prevent hazards and costly repairs. Legacy Air offers expert furnace maintenance services to keep your system efficient and safe. Reach out to Legacy Air for top-notch support and ensure your furnace’s longevity and reliability.