How to clean and replace a furnace flame sensor

If your furnace is acting up and you’re having trouble keeping it running smoothly, a dirty or faulty flame sensor could be the culprit. The flame sensor is a key part of your furnace’s safety system, ensuring that the gas valve stays off if no flame is detected. The good news? Cleaning or replacing it is a fairly simple DIY task that can get your furnace back in action. We’ll walk you through the steps to clean or replace your furnace flame sensor, so your heating system can keep you warm all season long. Let’s get started!

The info in this article might not cover every appliance model. While we aim to provide helpful advice for all sorts of DIY appliance repairs, different models and setups can mean different troubleshooting steps, error codes, part swaps, etc. For the most accurate guidance, we recommend checking the manufacturer’s documentation for your specific model or reaching out to us if you’ve got questions.

What is a furnace flame sensor?

The flame sensor in your furnace plays an important role in safety and proper operation. It detects whether the furnace burners have ignited by sensing the flame. If the sensor doesn’t detect a flame, it shuts off the gas supply to prevent a dangerous buildup. Over time, the flame sensor can become dirty or damaged, causing the furnace to malfunction—either shutting off too soon or not starting at all.

What to look for:

• The furnace starts but shuts off after a few seconds? This could be a sign that the flame sensor isn’t detecting the flame properly.
• The furnace cycles on and off frequently? A faulty or dirty flame sensor might be the culprit.

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Signs your flame sensor needs cleaning or replacement

Before jumping into cleaning or replacing the flame sensor, it’s important to know when it’s necessary. A faulty flame sensor can cause a variety of furnace issues, including poor heating and constant cycling.

What to look for:

• Frequent furnace shutdowns: If the furnace ignites but shuts off after a few seconds or minutes, this is often a sign that the flame sensor is dirty or malfunctioning. The sensor isn’t detecting the flame properly and is shutting off the gas to prevent a potential hazard.
• The furnace tries to ignite but doesn’t stay on: If your furnace repeatedly tries to ignite and then turns off without staying on, it’s usually due to the flame sensor not detecting the flame.
• Short cycling: Your furnace starts up and runs for a short period before turning off again, only to repeat the process. This cycle can happen several times and can be frustrating.
• Excessive gas smell: If you smell gas after the furnace tries to ignite but doesn’t stay on, it could be an indication that the gas valve is shutting off because the flame sensor isn’t doing its job.

Safety precautions before you get started

Safety comes first when working on any appliance. Here are some practical tips to help keep you safe:

• Always turn off the power by unplugging the appliance or switching off the circuit breaker before starting. This protects you from the risk of electric shock.
• Wear insulated gloves to protect your hands from sharp edges, pinching hazards, or debris.
• Take your time and work at a steady pace to avoid accidents or injuries.
• Make sure your workspace is well-lit so you can clearly see and access the parts you’re working on.
• Keep your area organized and free of clutter, and ensure children and pets are kept away from the workspace.
• Never handle internal parts with wet hands. Make sure both your hands and the work area are completely dry.
• Check your appliance’s user manual for specific instructions or safety guidelines related to your repair.
• Handle parts gently to prevent damaging the appliance or injuring yourself by using too much force.
• Wear safety glasses if you’re dealing with chemicals, dust, or large debris to protect your eyes.
• If the appliance was recently used, give it time to cool down before working on any heated components.
• Take photos or notes of wiring connections before disconnecting anything. This makes reassembly much easier.
• Avoid touching exposed wires or terminals. If you need to handle wires, use non-conductive tools or wear insulated gloves to reduce the risk of electrical shock.

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How to clean your furnace flame sensor

1. For safety, always turn off the furnace and disconnect the power supply before working on any parts.
2. The flame sensor is typically located near the furnace burners. It’s usually a small metal rod with one or two wires connected to it. If you’re not sure where it is, check your furnace manual for its location.
3. Use a screwdriver to remove any screws or fasteners securing the sensor. Once the screws are removed, carefully pull the sensor out of its mounting bracket.
4. Use a soft cloth or a fine wire brush to gently clean any dirt or buildup from the flame sensor. Be careful not to damage the sensor.
5. After cleaning, inspect the sensor for any signs of wear, such as cracks or corrosion. If it looks damaged, it may be time to replace it.
6. Once the sensor is clean, carefully reinstall it in the same position. Make sure it is securely fastened with the screws or fasteners.
7. After reinstalling the sensor, reconnect the power and turn the furnace back on. If the furnace operates normally and the blower shuts off when it should, the cleaning was successful.

How to replace a furnace flame sensor

1. Turn off the furnace and disconnect the power. Always ensure the furnace is off before working on any parts.
2. Locate the flame sensor. Find the flame sensor near the burners (refer to your manual for location details).
3. Remove the old sensor. Unscrew the sensor and disconnect the wiring.
4. Install the new sensor. Attach the wires to the new sensor and secure it with screws.
5. Test the new sensor. Reconnect the power, turn on the furnace, and check if it cycles properly. The blower should stop when the desired temperature is reached.
6. Monitor the furnace. Keep an eye on the furnace to ensure the new sensor is working well.

Read more: 12 simple space heater safety tips for this winter

Keep your furnace running 

Taking care of your furnace’s flame sensor is a simple task that can save you time and money. By keeping it clean or replacing it when needed, you’re making sure your furnace operates safely and efficiently. Remember, we’re here to help with any appliance repair needs you might have. Visit AppliancePartsPros.com for quality parts and expert advice.

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