Flame Sensor for Lennox Furnace: Troubleshooting, Replacement, and Maintenance

The flame sensor on a Lennox furnace is a small but critical safety device that confirms combustion is occurring and prevents gas flow if a flame is not detected. This article explains how Lennox flame sensors work, common symptoms of failure, step-by-step testing, cleaning and replacement guidance, troubleshooting tips, and maintenance best practices to keep a furnace running safely and efficiently.

Symptom	Likely Cause	Immediate Action
Furnace Ignites Then Shuts Off	Dirty Or Faulty Flame Sensor	Clean Sensor; Test; Replace If Needed
Hot Surface Igniter Works But No Flame	Ignition Or Gas Valve Issue	Inspect Igniter And Gas Supply; Call Pro If Needed
Repeated Lockouts	Intermittent Sensor Or Wiring Fault	Check Wiring, Replace Sensor

What Is A Flame Sensor And Why It Matters

The flame sensor is a thin metal rod positioned in the burner area that detects a flame by sensing ionized particles. When the furnace calls for heat and the igniter lights the burners, the flame sensor sends a small current to the control board to confirm combustion.

If the sensor Fails the control board will shut off the gas to prevent unburned fuel accumulation, causing repeated ignition attempts and lockouts. This makes the flame sensor a key safety and reliability component on Lennox furnaces.

Common Symptoms Of A Failing Lennox Flame Sensor

Recognizing symptoms early can save time and expense. Typical signs include: frequent furnace lockouts after ignition, burners igniting but shutting off within seconds, error codes or blinking LEDs on the control board, and improved operation after power cycling but recurring failures.

Other clues include visible corrosion on the sensor, carbon build-up on the rod, or intermittent operation that suggests poor electrical contact or a failing sensor element.

Tools And Safety Precautions Before Working On The Furnace

Before any inspection, turn off power to the furnace at the breaker and shut off the gas supply for safety. Use a multimeter, a putty knife or fine-grit sandpaper, nut drivers or a screwdriver, and a flashlight.

Safety Reminders: Avoid using harsh abrasives or flames near the gas valve. If uncomfortable with gas or electrical systems, contact a licensed HVAC technician. Follow Lennox manuals and local codes for safe practices.

How To Test A Lennox Flame Sensor

Testing usually requires a multimeter set to microamp or millivolt ranges. Access the flame sensor by removing the furnace access panel and locating the thin metal rod near the burners.

With the furnace energized and calling for heat, carefully measure the sensor circuit. A healthy flame sensor typically produces between 1 to 10 microamps. Readings significantly below this range indicate a weak or failing sensor or poor grounding.

Step-By-Step Cleaning Procedure

Cleaning often restores proper function. Turn off power and gas, remove the sensor by loosening its retaining screw or clip, and gently clean the metal rod with 400–600 grit sandpaper, emery cloth, or a non-abrasive pad until the metal looks bright.

Reinstall the sensor ensuring a snug mechanical and electrical connection. Restore power and gas, then observe several ignition cycles. Cleaning may fix many common issues, but repeat failures suggest replacement or wiring problems.

Replacement Guide: Choosing The Right Lennox Flame Sensor

When cleaning fails, replacement is the next step. Lennox furnaces use specific flame sensors designed for their models and burner geometries. Using the correct part ensures proper sensing and compliance with safety controls.

Locate the furnace model number on the rating plate and consult Lennox parts documentation or an authorized parts distributor to match the exact flame sensor part number. Cross-reference popular Lennox part numbers like XXXXXX only when verified by the model sheet or parts list.

How To Replace A Lennox Flame Sensor

Replacement steps are straightforward: shut off power and gas, remove the access panel, disconnect the single-wire connector from the sensor, and unfasten the mounting screw or clip. Install the new sensor in the same orientation and position to ensure proper flame detection.

Tighten connections, restore gas and power, and observe the furnace across several cycles to confirm stable operation. If the furnace still locks out, inspect wiring, ground continuity, and the control board for related issues.

Wiring And Grounding Considerations

Proper grounding is essential for accurate flame sensing. The flame sensor circuit relies on the furnace chassis as a return path. Loose or corroded connectors, missing ground connections, or damaged insulation can produce low microamp readings even with a healthy sensor.

Inspect wiring for frays or high-resistance connections and tighten terminal screws. Where grounding issues persist, test chassis continuity to neutral and correct any faults before replacing the sensor.

Interpreting Lennox Error Codes And LEDs

Lennox furnaces often use blinking LEDs or diagnostic codes to indicate ignition and flame sensing problems. A pattern of short or long flashes typically maps to specific faults like flame failure, pressure switch open, or limit switch trips.

Refer to the furnace’s data plate or service manual for the exact code definitions. Document the code pattern before making changes to aid troubleshooting or when contacting support or a technician.

When Cleaning Or Replacement Doesn’t Fix The Problem

If issues persist after replacing the flame sensor, consider other causes: a weak or failing gas valve, improper gas pressure, faulty ignitor, defective control board, blocked burners, or a malfunctioning flame rectification circuit.

At this point, professional diagnosis is recommended. A qualified HVAC technician can perform gas pressure tests, inspect control board outputs, and verify system-wide electrical integrity to locate the root cause.

Preventive Maintenance To Extend Flame Sensor Life

Regular maintenance reduces sensor failures. Annual or biannual furnace inspections that include burner cleaning, sensor inspection, and filter replacement minimize soot and carbon deposits that degrade the sensor’s performance.

Keep the burner compartment clean, change air filters on schedule, and ensure proper combustion air supply. Simple preventive steps often prevent misfires and extend the service life of the flame sensor and related ignition components.

Cost Expectations And Parts Availability

Replacement Lennox flame sensors are typically inexpensive as parts, though costs vary by model and supplier. Expect part costs to be modest; labor for professional replacement can increase total expense if a technician is required.

Genuine Lennox parts are available through distributors, HVAC wholesalers, and authorized service providers. Using OEM parts maintains warranty coverage and ensures compatibility with Lennox control boards.

DIY vs Professional Service: Decision Factors

Cleaning or replacing a flame sensor is often a manageable DIY task for someone comfortable with basic tools and safety procedures. However, if gas system work, control board diagnosis, or complex electrical testing is needed, a licensed HVAC technician is the safer choice.

Choose a professional when there are repeated lockouts, unclear error codes, suspected gas valve or control board issues, or when local codes require licensed service for gas appliance work.

Frequently Asked Questions About Lennox Flame Sensors

How Long Does A Flame Sensor Last?

With proper maintenance, a flame sensor can last many years, though exposure to soot, corrosion, and electrical wear will shorten lifespan. Regular inspection helps catch degradation early.

Can A Dirty Flame Sensor Cause Carbon Monoxide?

A dirty flame sensor itself does not produce carbon monoxide, but incomplete combustion from burner issues can. Any persistent combustion problem should be evaluated promptly for safety.

Is It OK To Use A Universal Sensor?

Universal sensors exist but must match length, mounting orientation, and electrical connection. Using an incorrect sensor can cause unreliable sensing. OEM or verified cross-reference parts are recommended.

Resources And Where To Find More Help

Refer to the Lennox furnace owner’s manual and service literature for model-specific diagrams, part numbers, and diagnostic code guides. Certified Lennox dealers and HVAC forums can provide additional support and parts sourcing.

If ongoing problems remain after following troubleshooting steps, contact a licensed HVAC professional with experience on Lennox systems for safe, compliant repairs.