Furnace Flame Sensor: Symptoms & Repair

Your furnace ignites, runs for a few seconds, and then shuts off — and it keeps doing it. In most cases, that pattern points to a dirty or failing furnace flame sensor, a $15 safety part that can shut down an entire heating system. This guide explains how the sensor works, how to confirm it’s the culprit, what cleaning and replacement cost in the US, and when it’s time to call a NATE-certified technician.

What Is a Furnace Flame Sensor?

A flame sensor is a short metal rod — usually 3 to 4 inches long and roughly 1/8 inch in diameter — mounted on a ceramic insulator and positioned so its tip sits directly in the burner flame. It’s wired to the furnace control board and acts as the system’s proof-of-flame safety device.

Modern gas furnaces use a hot surface igniter (HSI) to light the burners. The igniter glows, the gas valve opens, and the burners light — but the control board has no way of “seeing” that flame. That’s the flame sensor’s job. If it doesn’t confirm a flame within the trial-for-ignition window (typically 4 to 7 seconds), the board closes the gas valve immediately to prevent raw gas from accumulating in the heat exchanger and ductwork.

Under ANSI Z21.47 / CSA 2.3 (the standard covering gas-fired central furnaces) and NFPA 54, the National Fuel Gas Code, this flame-supervision function is mandatory on every automatically ignited appliance sold in the US.

How Flame Rectification Works

The sensor doesn’t use heat or a bimetallic strip. It uses a principle called flame rectification:

1. The control board applies a low AC voltage to the sensor rod.
2. When a flame is present, the ionized gases in the flame conduct electricity — but only in one direction.
3. That one-way conduction converts the AC signal into a tiny DC current, measured in microamps (µA).
4. The board reads that microamp signal. If it’s above the minimum threshold, the gas valve stays open. If it drops, the valve closes.

A healthy residential furnace typically produces 2 to 6 microamps of flame current, and most control boards require a minimum of about 1.0 µA (some specify 1.5 µA) to keep the burner lit. When the rod oxidizes, that signal drops below the threshold and the furnace shuts down — even though the flame itself is perfectly fine.

Flame Sensor vs. Thermocouple

Homeowners often confuse the two. A thermocouple generates millivolts (typically 250 to 750 mV) from heat and is used on older standing-pilot furnaces and water heaters. A flame sensor is a rectification rod used on electronic-ignition furnaces. If your furnace has a small glowing igniter and no standing pilot light, it has a flame sensor, not a thermocouple.

7 Symptoms of a Bad Furnace Flame Sensor

Because a faulty sensor mimics several other furnace problems, look for this specific cluster of symptoms:

• Burners light, then shut off after 5 to 10 seconds. This is the classic signature. The flame looks healthy, but the board never receives proof and closes the gas valve.
• Repeated ignition attempts. You’ll hear the igniter cycle and the gas valve click three times in a row, followed by silence.
• Lockout or a blinking fault code. After three failed tries, most boards enter a soft lockout for roughly 60 minutes. Common codes include Carrier/Bryant Code 14, Lennox Watchlight patterns, and Goodman/Amana LED flash sequences — check the sticker inside the blower door for your model’s chart.
• The blower runs but the air is cold. The indoor fan continues on its timed cycle even after the burners drop out, so you get cool air from the registers.
• Short cycling. The furnace heats for a few minutes, shuts down, and restarts — sometimes 3 to 10 times an hour.
• The furnace won’t restart without a power cycle. Flipping the breaker or the service switch resets the lockout, and the unit runs again briefly.
• Intermittent heat that gets worse over time. A marginally dirty sensor may work on mild days and fail on cold, damp mornings.

If your system is exhibiting these behaviors alongside other issues, see our guide to a furnace that isn’t heating for a broader diagnostic checklist.

What Causes a Flame Sensor to Fail?

In the field, the overwhelming majority of “bad” flame sensors are simply dirty, not defective.

Oxidation and carbon buildup. The sensor sits in a flame all winter. Over a season or two, it accumulates a thin layer of carbon and metal oxide that acts as an insulator, choking off the microamp signal. This is the number-one cause of flame-sensor failures in the US.

Moisture and high humidity. Damp basements and condensing (90%+ AFUE) furnaces produce more moisture in the burner compartment, accelerating corrosion on the rod.

Misalignment. If the sensor was bumped during service or the burner tray shifted, the rod may no longer sit in the flame. The tip should be immersed roughly 1/2 inch into the burner flame, aligned with the burner facing the igniter.

A cracked ceramic insulator. The ceramic base electrically isolates the rod from the furnace chassis. A hairline crack lets current leak to ground, killing the signal. A cracked insulator means replacement, not cleaning.

Poor electrical ground. Flame rectification depends on a solid ground path back to the control board. Loose green ground wires, corroded chassis connections, or reversed polarity at the outlet can all cause nuisance lockouts.

Dirty burners or low gas pressure. A lazy, yellow, lifting flame doesn’t ionize properly. Gas pressure should be set per the furnace data plate — typically 3.5 inches of water column (in. w.c.) for natural gas and 10 to 11 in. w.c. for propane on manifold pressure.

A failing control board. Less common, but if you measure strong microamps and the furnace still drops out, the board’s flame-sensing circuit may be the problem.

How to Test a Furnace Flame Sensor

Testing takes about 15 minutes with a multimeter that reads DC microamps.

Safety first: Turn off the furnace at the service switch, shut off the gas valve, and wait for the igniter to cool. Never touch the igniter — it’s fragile and can shatter.

1. Remove the sensor. It’s held by one or two 1/4-inch hex-head screws. Pull it straight out and inspect the rod.
2. Look for the tell. A white, gray, or black crust on the rod means cleaning is likely all you need. A cracked or crumbling ceramic base means replace it.
3. Set your meter to DC µA (microamps). Some meters require you to move the red lead to the µA jack.
4. Reconnect in series. Disconnect the sensor lead from the control board and connect one probe to the sensor terminal and the other to the board terminal, so the meter completes the circuit. On many boards you can unplug the flame-sense spade and clip directly to the pin.
5. Call for heat. Watch the reading once the burners establish flame.
6. Interpret the numbers. Below 1 µA is a fail. A clean, healthy reading is 2 to 6 µA. A reading that climbs slowly and then drops out signals a rod that needs cleaning.

If you’re uncomfortable working inside the burner compartment, or if the diagnostic points to a gas valve, igniter, or control board, it’s safer to book professional furnace repair than to keep guessing.

How to Clean a Furnace Flame Sensor

Cleaning is a 10-minute job and resolves the majority of flame-sensor lockouts.

1. Shut off power and gas.
2. Remove the sensor’s mounting screw and slide it out.
3. Wipe the rod with 0000-grade steel wool or a non-abrasive scouring pad. Polish gently until the metal is bright and shiny, working from the base to the tip.
4. Do not use coarse sandpaper, emery cloth, or a wire wheel. These remove the rod’s protective outer layer and can shorten its life. Never file or bend the rod — the tip position is calibrated.
5. Wipe away all residue with a clean, dry cloth. Any steel wool fibers left behind can short the sensor.
6. Inspect the ceramic insulator for cracks; replace the sensor if you find any.
7. Reinstall, snug the screw (don’t overtighten — you’ll crack the ceramic), reconnect the lead, and restore power and gas.
8. Run a heat cycle and confirm the furnace stays lit through a full cycle.

Realistic expectation: cleaning typically buys you a season. If you’re cleaning the same sensor twice a year, replace it — a new one costs less than most diagnostic fees.

Repair and Replacement Costs in the US

Flame sensors are one of the cheapest furnace repairs you’ll ever face.

A few cost variables to keep in mind:

• OEM vs. universal parts. Universal flame sensors run $10 to $25; brand-specific OEM sensors for Carrier, Trane, Lennox, and Rheem units can reach $40 to $80. Most are simple spade-terminal devices with no calibration, so a universal part is usually safe — but match the length, thread type, and mounting bracket.
• Warranty coverage. Most furnace manufacturers cover the flame sensor under a 5- to 10-year parts warranty, though labor is typically covered for only the first year. Keep your receipts and annual service records.
• Bundle it. If you’re already paying for a diagnostic visit, having the technician clean the sensor, check gas pressure, and inspect the heat exchanger adds very little to the bill.

If you prefer to source the part yourself, our furnace parts reference covers common components, part numbers, and compatibility.

When to Call a Professional

Call a licensed HVAC technician immediately if you notice any of the following:

• The smell of gas — leave the house and call your gas utility from outside.
• A yellow, orange, or lifting flame instead of a steady blue flame, which can indicate incomplete combustion or a venting problem.
• Soot deposits around the burners or in the flue.
• A cracked heat exchanger, which can introduce carbon monoxide into the supply air. This is a red-tag, do-not-operate condition. Every home should have UL-listed CO alarms on each level and outside sleeping areas.
• Zero microamps after cleaning, which points to wiring, grounding, or control-board faults.
• Repeated lockouts after a fresh sensor is installed.

Technicians diagnose these with a combustion analyzer, manometer, and microamp meter — tools that cost more than most homeowners want to buy for a one-time repair.

Preventing Flame Sensor Problems

A little routine care keeps flame sensors healthy:

• Schedule annual professional maintenance. A tune-up includes cleaning the flame sensor, checking gas pressure, verifying microamp readings, and inspecting the heat exchanger. The U.S. Department of Energy recommends professional servicing of heating equipment once a year.
• Change filters every 1 to 3 months. Restricted airflow raises temperatures in the burner compartment and accelerates oxidation.
• Keep the burner compartment dry. Fix basement humidity issues and make sure condensate drains on high-efficiency (90%+ AFUE) furnaces aren’t clogged.
• Verify grounding and polarity. A plugged-in furnace with a reversed hot/neutral or a missing ground will cause chronic flame-sense faults.
• Know your AFUE. Federal minimum efficiency for non-weatherized gas furnaces is 80% AFUE, while ENERGY STAR–certified models generally require 90% AFUE or higher in most regions. Higher-efficiency condensing furnaces produce more condensate, so their burner compartments need extra attention.

Frequently Asked Questions

How much does it cost to replace a furnace flame sensor? The part itself is inexpensive — typically $10 to $40 — but most homeowners pay $150 to $300 total for a professional service call that includes diagnosis, cleaning, and installation. If a technician simply cleans the existing sensor during a routine tune-up, expect to pay $100 to $200 for the visit.

Can I clean a furnace flame sensor myself? Yes. Cleaning a flame sensor is one of the few furnace repairs that is genuinely DIY-friendly. Shut off power and gas, remove the single screw holding the sensor, gently polish the metal rod with 0000 steel wool until it’s shiny, reinstall it, and restore power. Never use coarse sandpaper or emery cloth, and never bend the rod.

Will a furnace run with a bad flame sensor? No. The flame sensor is a safety device. If it can’t detect a flame, the control board shuts the gas valve within seconds to prevent unburned gas from accumulating. You may see the burners light for 5 to 10 seconds before shutting off, and after three failed attempts the furnace typically locks out for about an hour.

Bottom Line

A furnace flame sensor is a $10 to $40 part that protects your home from unburned gas — and it’s responsible for a large share of “furnace won’t stay lit” service calls. The signature symptom is burners that light and then die within 5 to 10 seconds, followed by three retry attempts and a lockout. Before you pay for a service call, shut off power, pull the sensor, and look at the rod: if it’s coated in white or black crust, ten minutes with 0000 steel wool will usually restore it to 2 to 6 microamps and get your heat back on. If the rod is clean, if the ceramic insulator is cracked, or if a meter shows near-zero flame current, the problem lies elsewhere in the ignition circuit — and that’s the point to bring in a licensed technician. Clean or replace the sensor once, keep up with annual maintenance, and you’ll likely never deal with a mid-winter lockout again.