If your heater lights, runs for a few seconds, and shuts off, the tiny metal rod near the burner often sits at the center of the story. That rod is the flame sensor. It proves that gas is burning. When it gets dirty or loses contact, the control board cuts the gas to avoid raw gas buildup. Safety comes first.
What a flame sensor in a furnace does day-to-day
The flame sensor’s job is simple: confirm flame after the gas valve opens. If the sensor sees flame, the board keeps the valve energized. If it does not, the board shuts the valve and tries again or locks out.
On modern direct-ignition furnaces, the sensor works with a hot surface ignitor or spark. The sensor sits in the fire and sends a tiny signal that the board can read. Older standing-pilot systems used a thermocouple, which is different. Most newer units rely on flame rectification instead of a pilot.
Where it sits and what it looks like
Look for a thin steel or nichrome rod that bends into the flame path, usually held by a white ceramic insulator and a single wire. It mounts with one screw near the burner face so the tip is washed by flame during each heat cycle.
Why furnaces use flame rectification
Gas flames conduct electricity in one direction better than the other. The control sends an AC signal to the rod. When flame is present, charged particles in the fire pass a small DC current from the rod, across the flame, into the grounded burner and back to the board. That one-way current proves flame.
| Topic | What it means | Why it’s used |
|---|---|---|
| Role | Proves flame so gas keeps flowing | Prevents raw gas release |
| Location | Rod tip in the burner flame | Fast detection each cycle |
| Signal type | DC microamp current from flame | Clean, stable proof signal |
| Board reaction | Keeps or stops the gas valve | Stops unsafe firing |
| Needs to work | Clean tip, solid ground, correct rod position | Steady heat and safe starts |
How a furnace flame sensor works
Right after ignition, the board looks for a flame signal. A healthy signal often falls between about 0.5 and 10 microamps depending on model, with many units living in the 2–6 microamp range. Each brand publishes its own target window.
No signal means the board shuts the valve within seconds. Many furnaces will retry a few times, then enter soft lockout to clear gas and protect the home. That cycle looks like short bursts of flame followed by rapid shutoff.
Signs your flame sensor needs care
The most common clue is the burner lights, then drops out. You may hear the fan start, feel a quick blast of heat, and then nothing. Some units flash a code for flame loss or no flame sensed.
Grime on the rod, weak grounding, or a mis-aimed tip are usual triggers. Rust flakes, moisture, or a cracked insulator can also break the signal. Any time you remove the burner or swap an ignitor, recheck the rod position.
Safe cleaning and quick checks
Many false trips come from a dirty rod. You can try a gentle clean on a cool system if you are comfortable working around gas appliances. If not, call a licensed tech.
Tools you may need
- Phillips or nut driver for the mounting screw
- Fine abrasive pad or fine steel wool
- Clean lint-free cloth
- Multimeter with a DC microamp scale
- Work light and gloves
Steps for a careful clean
- Switch off power at the service switch and shut the gas control if the manual calls for it. Wait for the burners to cool.
- Remove the access panel. Photograph the rod location so you can match it later.
- Loosen the screw, slide the sensor out, and unplug the wire by the connector—not by pulling the wire jacket.
- Buff the rod tip with the pad until the surface looks bright. Do not sand the ceramic insulator.
- Wipe the rod clean to remove residue. Check the porcelain for cracks and the wire for loose terminals.
- Reinstall the rod so the tip sits in the flame path just like the photo. Tighten the screw snug, not overtight.
- Restore power and gas, then call for heat. Watch the ignition sequence.
- If the burner still drops out, test microamps per the manual or schedule service.
When to stop and call a pro
Stop if you smell gas, see scorch marks, or find cracked ceramic. Stop if the wiring looks damaged or the board shows unexpected codes. Stop if the unit retries many times. Those signs call for a trained technician.
Troubleshooting cheatsheet for a furnace flame sensor
| Symptom | Likely cause | What to try |
|---|---|---|
| Burner lights then shuts off in 3–10 seconds | Contaminated rod or weak ground | Clean the rod and verify ground path to the board |
| No light at all, inducer and ignitor run | Board does not see flame signal | Check rod position, wire, and microamp reading |
| Random dropouts during a long call for heat | Flame lifting or draft issue | Verify gas pressure and venting; have a tech test combustion |
| Rod reads near zero microamps with flame | Rod not engulfed in flame or open circuit | Aim the tip into flame; repair wire or replace rod |
| Rod reads high then falls over time | Oxidation building on tip | Clean the rod; check for moisture or rust in the burner area |
Preventive habits that help the sensor
Keep the combustion area clean and dry. Change filters on time so airflow stays in spec and the flame stays steady. During yearly service, ask for a flame signal check, a burner cleaning, and a ground-path inspection.
Related safety and standards
Gas appliances follow codes and product standards that reduce fire and gas hazards. Service manuals describe the exact test points for your model. For general furnace care and safe operation, see the U.S. Department of Energy’s guidance on maintenance for furnaces and boilers. Codes for fuel-gas appliances are maintained by NFPA under the National Fuel Gas Code. Manufacturers such as Carrier publish plain-language parts guides that explain what each part does, including the flame sensor.
Replacement basics and what to expect
If cleaning does not restore a stable signal, the sensor may be worn or the circuit may have other faults. Sensors are model-specific. Many look alike, but the length, bend, and mounting tab vary. Match the exact part number or follow the maker’s cross-reference. A tech will also check polarity, grounding, and the control board before calling the job done.
Practical points you can use
A clean flame sensor protects your home by proving the flame every cycle. Keep the tip in the fire, keep the ground path tight, and keep the rod bright. If short cycling continues after a careful clean, bring in a pro and ask for a microamp test while the burners run.
Write the date of your last clean on a small tag inside the panel. That reminder helps you track service and spot patterns if lockouts return next season during each season.