Step-by-Step Guide to Stop Furnace Short Cycling: Flame Sensor Cleaning

Short cycling, a common issue where a furnace turns on briefly before shutting off, can be frustrating. However, the solution is often straightforward and involves cleaning the furnace flame sensor. This maintenance task can help resolve the problem without the need for a professional technician. To better understand short cycling, it's important to understand the sequence of operations when a furnace starts up. You can click on any of the images below for an enlarged view.

The sequence of operations when a furnace starts up.

Technician pointing to inducer motor in the furnace, initiating the heating process.

1. The inducer motor initiates the heating process. This first image demonstrates step one: the inducer motor initiating the heating process. It's the component I'm pointing to here. This motor is responsible for beginning the sequence by ensuring exhaust gases are safely vented outside your home. It's important to know this step because the inducer motor must be operational for the furnace to work correctly. Understanding this is key before we move on to addressing the short-cycling issue, which will be shown in the subsequent images and instructions.

Service tech indicating pressure switch for airflow verification in the furnace.

2. Airflow sensors confirm proper ventilation. In this second step of our furnace start-up sequence, I'm pointing to the pressure switch, a safety device that checks for proper airflow through the furnace. Before the furnace can ignite, this switch ensures that the inducer motor pulls enough air to remove exhaust gases safely. If this switch isn't working, the furnace won't start, preventing any potential safety issues.

Ignitor ready to heat gas burners, shown by furnace repair expert.

3. The ignitor heats up to ignite the gas burners. Now, in the third step of our furnace start-up sequence, I'm showing you the ignitor, which is responsible for lighting the burners. It's an electronic component that gets very hot to ignite the gas coming into the furnace. When the pressure switch confirms that it's safe to proceed, the ignitor begins the actual heating process. It's important for this piece to work properly because if it doesn't, the furnace won't be able to produce heat.

Hand pointing to the gas valve, crucial for controlling fuel flow to the furnace.

4. The gas valve opens to allow fuel flow. Continuing with our furnace start-up sequence, we've arrived at the fourth step where I'm pointing to the gas valve. This critical component controls the flow of gas to the furnace burners. Once the ignitor is hot enough, the gas valve opens to allow gas to flow through, which the ignitor then lights to start the heating process.

Flame sensor detection demonstrated, key to furnace operation and safety.

5. The flame sensor detects the presence of a flame. I'm focusing on the flame sensor in the fifth and final step of our furnace start-up sequence. This small but crucial component has a big job: it confirms to the furnace that the burners have successfully lit, and a flame is present. When the gas valve releases gas, and the ignitor lights it, the flame sensor detects the heat from the flame, ensuring the furnace continues to operate. If the sensor doesn't detect a flame, it will shut down the furnace to prevent unburned gas from accumulating, which is a safety measure to avoid potential gas leaks. Understanding the role of the flame sensor is important as a dirty sensor can cause the furnace to shut off prematurely, a common issue known as short cycling.

Easy Steps to Clean Your Flame Sensor and Fix Your Furnace

The flame sensor plays a crucial role in maintaining the furnace's operation by verifying the presence of a flame. When this sensor becomes dirty with debris like dust and combustion particles, it fails to detect the flame signal correctly. Consequently, the furnace shuts down as a safety precaution, leading to short cycling. Fortunately, cleaning the flame sensor can often resolve this issue.

Follow these steps to clean the flame sensor

Turning off the power switch to ensure safety before furnace repair.

1. Turn off the power supply to the furnace for safety. As we start our DIY repair, the first step is to ensure safety by turning off the power to the furnace. Here, I'm doing just that. I'm at the power switch, which is usually located on or near the furnace. It's important to switch this off before you begin any work on your furnace. This prevents any electrical accidents and makes sure you can safely handle the components inside.

Removing furnace cover to access internal components for DIY repair.

2. Remove the outer panel to access the sensor. Now that the power is off, removing the furnace's outer cover is next. This will give you access to the internal components, like the flame sensor we'll focus on. I'm carefully removing the cover here. Make sure to keep all screws and covers in a safe place so you can put everything back together after you've finished the repair.

Identifying flame sensor near burners for cleaning, crucial maintenance step.

3. Identify the flame sensor, typically connected by a single wire. With the cover off, we’re moving into the core of the furnace to locate the flame sensor. Here, I'm pointing to where you'll usually find the flame sensor within the furnace. It's typically mounted near the burners and has a wire leading to it. Make sure you identify it correctly before proceeding to ensure you’re working on the right component. The flame sensor is a small, rod-like part we’ll focus on cleaning to address any short-cycling issues.

Cleaning flame sensor with scotch brite pad to prevent short cycling.

4. Clean the sensor using steel wool or a Scotch pad. Once you've located the flame sensor, the next step is to clean it. I use a scotch brite pad to gently rub the rod, removing any buildup or corrosion. This buildup can cause the sensor to fail to detect the flame correctly, leading to furnace short-cycling. Be sure to clean it gently to avoid any damage, and don't touch the sensor with your fingers, as oil from your skin can cause further issues. After you finish cleaning, the sensor should have a clean, shiny metal appearance, indicating it can function properly again.

Reinstalling cleaned flame sensor to maintain efficient furnace operation.

5. Reinstall the sensor, close the furnace, and restore power. After the flame sensor is cleaned, it's time to reinstall it. In this image, I use a screwdriver to secure the flame sensor back into its proper place. It's important to make sure it's fitted correctly, and the connections are tight to detect the flame once the furnace is running again accurately. With the sensor back in place, the next steps will be to close up the furnace by replacing the cover, ensuring all screws are back in place, and then finally, restoring power to the system. Once power is back on, the furnace should be ready to run smoothly with no more short-cycling issues, thanks to a clean flame sensor.

While this process is generally straightforward, some furnace models, especially high-efficiency ones, may present difficulty accessing the sensor due to sealed burner chambers or sensor placement. In such cases, additional steps or component removal may be necessary.

By cleaning the flame sensor, you can restore proper functionality to your furnace and prevent short cycling issues. This simple maintenance task can be completed with basic tools and minimal time investment, offering a cost-effective solution to common heating problems.
If you encounter persistent furnace issues, have further questions, or want AC Repair San Ramon to repair your furnace, please ask for assistance. Ensure safety protocols are followed when working on furnaces to prevent accidents and ensure a successful repair.

Headshot of Pat Rosen, an HVAC contractor servicing the San Ramon East Bay area of Contra Costa County.

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