The telltale symptom of a bad or dirty flame sensor is unmistakable: your furnace ignites, burns for exactly 3–10 seconds, then the flame goes out and the furnace shuts down. After 3–4 failed ignition attempts (spaced about 60 seconds apart), the furnace locks out completely. This is the #1 furnace service call in the U.S., and in about 80% of cases, the fix is simply cleaning the sensor with fine-grit sandpaper — a 10-minute DIY job that saves a $150–$200 service call.
What Does a Flame Sensor Do?
The flame sensor is a safety device — a thin metal rod (usually stainless steel or nickel alloy) that extends into the burner flame. Its job is to confirm that gas has successfully ignited. If the gas valve opens but the gas doesn't ignite, raw gas could fill the combustion chamber — creating an explosion risk. The flame sensor prevents this by verifying ignition within seconds.
Here's how it works technically: when the flame contacts the sensor rod, it creates a small electrical current (measured in microamps, typically 1–6 µA) through the flame's ionization of the air. The control board monitors this current — if it doesn't detect the expected signal within 3–7 seconds, it closes the gas valve.
A dirty sensor has a coating (carbon, oxidation, corrosion) that insulates the rod from the flame, preventing the electrical signal from passing through. The flame IS there, but the sensor can't "see" it.
5 Symptoms of a Bad Flame Sensor
The key distinguishing symptom: The burner lights (you can see the flame through the sight glass) but shuts off within 3–10 seconds. If the burner never lights at all (no flame visible), the problem is more likely a bad igniter, gas valve, or gas supply issue — not the flame sensor.
Where to Find the Flame Sensor
The flame sensor is located inside the burner compartment — you'll need to remove the furnace access panel to see it. It's a thin metal rod (about 2–3 inches long) mounted to a porcelain or ceramic insulator base, held in place by a single 1/4-inch hex screw. It's usually on the opposite end of the burner assembly from the hot surface igniter.
Look for a single wire connected to the sensor's base — this wire carries the microamp signal back to the control board.
DIY Flame Sensor Cleaning: Step-by-Step
Safety first: Turn off the furnace at the power switch AND close the gas valve before working on the flame sensor. Wait at least 5 minutes for any residual gas to dissipate and for the sensor to cool down.
What you need:
- Fine-grit sandpaper (220-grit or emery cloth) or a dollar bill
- 1/4-inch nut driver or socket
- Clean cloth
Steps:
- Turn off the furnace at the power switch and close the gas valve.
- Remove the access panel (lower panel on most furnaces).
- Locate the flame sensor — thin metal rod with a single wire, usually to the right or left of the burner assembly.
- Disconnect the wire from the sensor base (it usually pulls off or has a small connector).
- Remove the mounting screw (single 1/4-inch hex screw).
- Gently slide the sensor out of its mounting bracket.
- Clean the metal rod with fine sandpaper or emery cloth. Rub gently along the length of the rod, rotating it to clean all sides. You're removing the carbon and oxidation layer — the rod should look shiny when done.
- Wipe with a clean cloth to remove any dust or particles.
- Reinstall the sensor — slide it back into the bracket, tighten the mounting screw, and reconnect the wire.
- Restore gas and power and test the furnace.
The entire process takes 10–15 minutes. The furnace should ignite and stay running on the first try if the flame sensor was the issue.
In a pinch, a dollar bill works. If you don't have sandpaper, fold a dollar bill and rub the metal rod between the folds. The slightly abrasive paper currency removes light carbon deposits. It's not as thorough as sandpaper but can get you through the night.
When to Replace (Not Just Clean)
Clean the sensor first — it solves the problem about 80% of the time. If cleaning doesn't help, or if the sensor needs cleaning more than twice per heating season, consider replacing it.
Signs the sensor needs replacement:
- Cleaning doesn't restore normal operation
- The metal rod is visibly corroded, pitted, or bent
- The porcelain insulator is cracked
- The sensor has been in service 7+ years (typical lifespan: 5–10 years)
Replacement cost: The sensor itself costs $8–$25 as a part. Professional replacement (including service call) costs $120–$250. If you're comfortable with the DIY cleaning process, replacing the sensor is essentially the same procedure — remove the old one, install the new one.
How to Test a Flame Sensor (For the Technical)
If you have a multimeter, you can measure the flame sensor's microamp signal to confirm it's the issue:
- Set your multimeter to the DC microamp (µA) range.
- Disconnect the flame sensor wire from the control board.
- Connect the multimeter in series between the sensor wire and the control board terminal.
- Run the furnace and read the microamp signal while the flame is on.
Expected readings:
- 1.0–6.0 µA → Sensor is working properly
- 0.5–1.0 µA → Sensor is marginal; clean or replace soon
- Below 0.5 µA → Sensor is bad; clean immediately; replace if cleaning doesn't improve
Most homeowners don't need this test — the symptom pattern (lights then shuts off in seconds) combined with a successful cleaning is sufficient confirmation.
Real-World Examples
Example 1: The Martins — DIY Fix in 10 Minutes (Columbus, OH) Their 5-year-old Rheem furnace would light, run for about 4 seconds, then shut off. After 4 failed attempts, it locked out. Mr. Martin searched online, identified the flame sensor symptom, and cleaned it with 220-grit sandpaper. The furnace ran perfectly on the first try after cleaning. Time spent: 12 minutes. Cost: $0. He now cleans it every fall as part of his pre-season checklist.
Example 2: The Petersons — Paid for a Service Call (Minneapolis, MN) The Petersons' furnace started failing on the coldest night of January (-5°F). Not comfortable with DIY, they called an HVAC company for emergency service. The technician cleaned the flame sensor — total bill: $195 ($125 emergency service fee + $70 repair). A 10-minute fix that Mr. Martin did for free. There's no wrong answer — just different comfort levels with DIY.
Example 3: The Williamses — Needed Replacement (Atlanta, GA) Their 9-year-old Carrier furnace had the flame sensor cleaned twice in one winter. The third time it failed, the technician found the rod was visibly pitted and corroded. A new sensor ($18 part, $150 total including labor) permanently fixed the issue. At 9 years old, the sensor had simply reached the end of its useful life.
Key Takeaways
- The classic symptom: Burner ignites, burns 3–10 seconds, then shuts off. Repeats 3–4 times, then locks out.
- 80% of flame sensor issues are fixed by cleaning the rod with fine sandpaper — a free, 10-minute DIY task.
- If the burner never lights at all, the problem is likely the igniter or gas supply, not the flame sensor.
- The sensor is easy to find: thin metal rod with one wire, single mounting screw, inside the burner compartment.
- Replace the sensor if cleaning doesn't work, if the rod is corroded/pitted, or if you're cleaning it more than twice per season.
- Replacement parts cost $8–$25. Professional service costs $120–$250.
- Include flame sensor cleaning in your annual fall furnace tune-up to prevent mid-winter failures.
Frequently Asked Questions
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