How do I clean a furnace flame sensor?
To clean a furnace flame sensor in a Minnesota home, shut off power and gas, let it cool, remove the rod (one wire, one screw), rub it lightly with a fine abrasive pad, reinstall it and watch a cycle. Stop if you smell gas. A furnace tune-up is $129.
Key points
- Power off at the furnace switch or breaker and gas off before you touch anything.
- The classic symptom is burners that light, run a few seconds, shut off, then try again.
- Use a fine, clean abrasive pad and a light touch; don't grind on the rod or stress the porcelain.
- Stop and call if you smell gas, see a cracked insulator, or the sensor coats up again within weeks.
- A tune-up is the natural time to clean the sensor and read its flame signal.
Cleaning a flame sensor takes about 20 minutes: switch off the power and the gas, let the burner area cool, take out the single screw and wire, rub the rod lightly with a fine abrasive pad until the dull film is gone, and put it back. The rod sits in the burner flame and tells the control board the gas lit. When film weakens that signal, the board closes the gas valve a few seconds after ignition. It's a reasonable homeowner job if you work carefully and know when to stop.
Is it safe to clean a flame sensor yourself?
Before you open the furnace:
- Cut the power. Turn the furnace off at its service switch, which usually looks like a light switch on or near the furnace, or at the breaker. Raise the thermostat to confirm nothing happens.
- Turn off the gas to the furnace. Most gas valves have a knob or small lever marked on and off. You can also close the manual shutoff on the gas pipe beside the furnace; the handle crosswise to the pipe means off.
- Let it cool. The burners, the sensor and the metal around them get hot. Wait until the burner area is cool to the touch.
- Stay within your lane. You're removing one screw and one wire. Don't take apart burners, the gas valve or anything with threaded gas pipe.
If your furnace has a standing pilot light that stays lit all the time, it uses a thermocouple, not a flame sensor, and this guide doesn't apply.
Is it really the flame sensor?
Watch a startup through the small viewing window in the furnace door, if yours has one, or just listen closely. A modern furnace runs the same sequence every time: the inducer fan starts, the igniter glows or sparks, the gas valve opens, and the burners light. Within a few seconds the flame sensor has to report a flame, or the board closes the gas valve and tries again. After several failed tries, many furnaces lock out and try again later.
A dirty sensor looks like this: burners light, burn for a few seconds, go out, and the cycle repeats. Other failures look different. If the igniter never glows, see our furnace won't ignite guide. If the burners run for several minutes and then shut off, that's usually overheating or airflow, covered in our post on furnace short cycling. Many furnaces also flash a code on the control board's light; the chart is usually on the inside of the door.
| What you see | Likely cause | What you can do | Who handles it |
|---|---|---|---|
| Burners light, go out within seconds, furnace retries | Dirty or poorly grounded flame sensor | Clean the sensor | You, then a technician if it returns |
| Inducer runs, igniter never glows | Igniter, pressure switch or a blocked vent | Check the outdoor PVC pipes for snow and ice | A technician if they're clear |
| Burners run several minutes, then shut off | Overheating from a clogged filter or closed registers | Replace the filter, open registers | A technician if it repeats |
| Nothing happens at all | Power, the door safety switch, the thermostat or a tripped condensate switch | Check the furnace switch, breaker and door | A technician |
| Yellow, lazy flames or soot | A combustion problem | Shut the furnace off | A technician |
In winter, a blocked intake or exhaust pipe is the Minnesota version of the second row; our guide to clearing a frozen furnace vent covers it.
What do you need to clean a flame sensor?
- A 1/4-inch nut driver or a screwdriver, depending on the screw
- A clean, fine abrasive pad (the non-soapy scrubbing kind) or very fine emery cloth
- A flashlight and a dry paper towel
- Your phone, to photograph the wire and sensor before removing them
How do you clean a flame sensor step by step?
- With power and gas off, remove the furnace's front door. Many furnaces have a door switch that cuts power when the door comes off; don't rely on it, but put the door back before testing.
- Find the sensor. It's a thin metal rod, often bent, with a white porcelain insulator at its base and a single wire attached. It's mounted in the burner area so the tip sits in a flame, often at the end of the burner row away from the igniter. Take a photo.
- Remove it. Pull the wire off its connector by the connector, not the wire. Remove the single mounting screw and slide the sensor out. Hold it by the metal base, and don't bend the rod or knock the porcelain.
- Clean the rod. Rub the metal lightly until the dull film is gone and the metal looks clean. Don't sand it hard, and don't use sprays, solvents or water. Wipe off the dust.
- Inspect it. A cracked or chipped porcelain insulator, or a rod that's pitted, bent or burned thin, means replacement, not cleaning.
- Reinstall. Slide it back in the same position, tighten the screw snugly, push the wire back on, and replace the door.
- Test. Turn the gas back on, then the power, and raise the thermostat. The burners should light and stay lit, and the blower should start about a minute later.
When should you stop and call a furnace technician?
Close it up and call a technician if you see or notice any of these:
- Soot or black streaks around the burners, or on the furnace filter (our post on black furnace filters explains why that matters)
- Yellow, lazy or lifting flames instead of steady blue ones, or flame rolling out of the burner opening
- Melted wire insulation, rust flakes or water dripping onto the burners
- A cracked porcelain insulator
- The furnace still locks out after cleaning
- The sensor needs cleaning again within a few weeks, which points to a combustion or air-supply problem
- Your carbon monoxide alarm goes off; get out first and call 911 (our guide to CO alarm placement covers the alarms)
Don't keep resetting a furnace that keeps failing. If it's a cold night and the heat won't stay on, call for furnace repair.
Why does a furnace flame sensor get dirty?
The film builds up from normal combustion over a season, and a Minnesota furnace runs from fall into spring with a lot of starts. Dust and chemical vapors in the combustion air, like laundry products or remodeling dust near the furnace, can speed it up.
A technician does more than clean it. With a meter, a technician reads the flame signal in microamps and compares it with the manufacturer's minimum, to learn whether a clean sensor is actually doing its job or whether the problem is the flame, the ground connection or the board. A fall tune-up is a good time to have that reading taken, and our furnace inspection checklist lists what else a visit should cover.
What if the furnace locks out on a January night?
A weak sensor tends to show itself when the furnace goes from a few starts a day to dozens, in the first cold week of fall or the first real cold snap. If the furnace locks out on a subzero night, one reset is fair: turn the furnace switch off for 30 seconds and back on, and watch a cycle. If it fails again, stop resetting, open the cabinet doors under sinks on outside walls, and call. Our no-heat checklist covers what to check while you wait and how to protect pipes. No-heat calls get priority. Our page on furnace repair in Minneapolis covers older furnaces in the city.
What does flame sensor cleaning or replacement cost?
- Cleaning is part of our furnace tune-up, which is $129, or $219 with the AC.
- A new flame sensor: national cost guides such as HomeGuide put replacement at $80–$250. Technicians often replace it along with the igniter, which HomeGuide puts at $100–$250 nationally.
- A control board, when the board itself can't read a good flame: HomeGuide's typical national range is $300–$650.
- With us, a repair visit carries a $79 trip charge, waived when you approve the repair. You get a written, flat-rate price before work starts, and completed repairs carry a 1-year parts-and-labor warranty.
Our furnace repair cost guide covers the other common parts.
What to do next
If cleaning fixed it, put a reminder in your calendar for next fall. If it didn't, or you'd rather not open the furnace, call 612-429-3465. A furnace tune-up is the easy time to have the sensor cleaned, and you can ask us about a maintenance plan if you want it handled every year.
Common questions about maintenance
Can a bad ground make a clean flame sensor fail?
Yes. The control board needs a good ground path to read the flame, so a clean sensor with a poor ground or loose connection can still lock the furnace out. A technician checks this with a microamp reading of the flame signal and compares it with the manufacturer's minimum, which a visual check cannot do.
How often should a flame sensor be cleaned?
Once a year is typical, and it's part of a normal fall furnace tune-up. If a sensor coats up again within a few weeks, cleaning isn't the fix. That points to a combustion problem or contaminated combustion air, which a technician should track down.
Should I replace the flame sensor instead of cleaning it?
Only when the porcelain is cracked or the rod is pitted, bent or burned thin. A sensor that is only dull with film cleans up, and a sensor that coats up again within weeks points to a combustion or air-supply problem rather than a bad part.
What should I use to clean a flame sensor?
A fine non-soapy abrasive pad or a strip of very fine emery cloth, used with a light touch until the dull film is gone and the metal looks clean. Skip sandpaper, steel wool, sprays and water. Rough abrasives scratch the rod and shorten its life, and moisture left on it can hide a good flame signal.
Sources
- HomeGuide: furnace repair costs homeguide.com