Lennox Furnace Flame Sensor Problems: Fixes & Costs

Last updated Sep 29, 2026 · By fixme.vip Editorial

Lennox furnace flame sensor problems: what it means

Lennox furnaces use a flame-sensing rod to confirm that the burners actually lit after the igniter fires. The rod generates a tiny electrical current (measured in microamps) when it is bathed in flame. The control board reads that current and, if it is too low or absent, the board assumes there is no flame and closes the gas valve for safety.

“Flame sensor problems” is a category rather than a single Lennox code number. Depending on your control board, the same underlying fault can surface as low flame current, flame sensed out of sequence, or a full ignition lockout after repeated retries. Lockout and reset behavior varies by control board: many Lennox boards enter a soft lockout and retry automatically after about an hour, while others require a power interruption before they will try again. Check the diagnostic label inside the furnace door for the behavior and code list that apply to your specific unit.

If your furnace lights for a few seconds and then shuts off, repeatedly, a dirty or failing flame sensor is one of the most common culprits.

Lennox furnace flame sensor problems: common causes, ranked by probability

1. Dirty or oxidized flame sensor rod — the most common cause by far. A carbon or silica film on the rod insulates it and drops the current the board can read.
2. Weak flame current from burner or gas issues — misaligned burners, a low or dirty flame, or partial blockage can starve the rod of clean flame contact.
3. Corroded or loose sensor wiring / connector — a poor connection between the rod and the control board weakens or interrupts the signal.
4. Cracked or worn flame sensor rod — the ceramic insulator can crack or the metal rod can degrade over years of heat cycling.
5. Failed or drifting control board flame-sense circuit — less common; the board reads flame current incorrectly even when the sensor is fine.
6. Poor furnace ground — flame sensing relies on a good ground path; a degraded ground reduces measured microamps.

Lennox’s own troubleshooting flow generally has technicians measure flame current and clean/inspect the sensor first before condemning the board.

How to fix Lennox furnace flame sensor problems: safe checks first

These are the only steps a homeowner should do. Anything inside the cabinet is technician work.

• Check the thermostat: set it to Heat and a few degrees above room temperature. Replace the batteries if it uses them.
• Replace a dirty air filter: a clogged filter can cause overheating and short cycling that mimics flame-sense faults.
• Check the breaker and furnace switch: make sure the furnace breaker is on and the service switch (looks like a light switch) near the unit is on.
• One reset only: if the furnace is locked out, cycle power at the breaker once and let it attempt a full ignition. If it locks out again, stop and call a pro — do not keep resetting.
• Look at supply and return vents: make sure they are open and not blocked by furniture or rugs.
• Condensate line (high-efficiency models only): this does not cause flame-sense faults. It is only worth checking if your furnace is reporting a pressure-switch or drain fault rather than a flame dropout — in that case, clear any obvious standing water at the drain.

Do not open the burner compartment, clean the sensor, or touch any wiring — those are gas-side technician tasks.

How a technician will diagnose it

A qualified tech will typically:

1. Put a meter in series with the flame sensor and measure flame current in microamps during a call for heat. The minimum acceptable flame signal for your board is printed on the unit’s wiring diagram and installation manual, and the reading has to be judged against that figure — not a generic number.
2. Remove and inspect the sensor rod for carbon buildup, cracks in the ceramic, and burn-through.
3. Clean the rod with a non-abrasive pad or fine steel wool — never emery cloth or sandpaper, which can leave residue or glaze the rod — then re-measure current.
4. Inspect the sensor wire and connector for corrosion, chafing, and a solid connection at the board.
5. Verify burner flame quality and alignment so the rod sits properly in the flame.
6. Confirm furnace ground, since poor grounding lowers readable current.
7. If the sensor is clean, wired well, and the flame is good but current is still wrong, they check the control board’s flame-sense circuit.

If a tech quotes a board replacement without first measuring flame current and cleaning the sensor, ask why.

Symptom, cause and what to do

Lennox furnace flame sensor problems repair cost

Honest US ranges (parts + labor; regional and model variation applies):

• Diagnostic / service call: $80–$150
• Flame sensor cleaning: $80–$180 (often bundled into a tune-up)
• Flame sensor rod replacement: $150–$300 including labor
• Sensor wiring / connector repair: $100–$250
• Ground correction: $100–$250 depending on the fault
• Control board replacement (if flame-sense circuit fails): $400–$800+

Cleaning is the cheapest and most common fix. Replacing the whole control board should be a last resort after flame current has been measured.

Related pages

• Lennox Furnace Low Flame Current: Causes, Fixes & Costs — the low-microamp symptom most of these problems generate.
• Lennox Furnace Flame Sensed Out of Sequence: Causes & Fixes — flame detected at the wrong point in the cycle.
• Lennox Furnace Ignition Lockout: Causes, Fixes & Costs — where repeated flame-sense failures end up.
• Lennox Furnace Keeps Shutting Off: Causes, Fixes & Costs — broader shutdown troubleshooting.
• Lennox Furnace Short Cycling: Causes, Fixes & Costs — related short-cycle behavior.
• Lennox Furnace Won’t Ignite: Causes, Fixes & Costs — when the burners never light at all.