Awaiting expert review. This page covers safety or regulatory rules that change over time. It is kept out of search results until a qualified reviewer has checked it; the sources below are dated so you can verify each point.
Quick answer
A furnace flame sensor is a thin metal rod in the burner flame that lets the furnace control confirm the burners are actually lit; if it does not detect flame within seconds, the control shuts off the gas.
It works by flame rectification: a tiny current flows through the flame to the rod. A coated or misplaced sensor reads too little current, so the furnace lights, then shuts off.
Quick facts
The key facts about Furnace Flame Sensor, with sources:
- What it is
- A probe on the burner or manifold assembly 1
- How it works
- Flame rectification proves flame is present 1
- Typical signal
- 1 to 6 microamps at 115 volts after cleaning, on one Bosch furnace 1
- Common failure
- A nearly invisible coating insulates it and drops the signal 1
- Lockout
- After three failed ignition attempts on one Bosch model 1
- Example blink code
- Trane IFC: 8 red flashes = low flame sense signal 2
- Who services it
- Qualified service technicians only, per the maker 1
Key takeaways
- The flame sensor is a safety device: no proven flame, no gas.
- A dirty sensor is a common reason a furnace lights and then shuts off within seconds 1.
- Bosch's manual says cleaning is for qualified service technicians only 1.
- Blink codes on the control board often point to it; codes differ by brand 2 1.
- If you smell gas, leave the building and call the gas supplier from outside 1.
What the flame sensor does
A gas furnace must never pour gas into a cold burner. So after the igniter glows and the gas valve opens, the control board waits a few seconds for proof that flame is there. The flame sensor provides that proof.
Bosch describes it as "a probe mounted to the burner/manifold assembly which uses flame rectification to determine the presence or absence of flame" 1.
A small voltage on the rod pushes a tiny current through the flame to the grounded burner.
Flame conducts current better in one direction, and the control reads that one-way current as "flame present."
If the current is too low, the board assumes the flame went out and shuts the gas valve. It may try again, and after several failures it locks out. On one Bosch model, lockout follows three failed ignition attempts 1.
Where it fits in a normal start
What a flame sensor failure looks like
- You hear the inducer start, then the igniter click or glow.
- The burners light for a few seconds.
- The burners shut off, and the inducer may keep running briefly.
- The furnace tries again, sometimes several times.
- It locks out and the control board LED blinks a code.
This pattern can also come from wiring, grounding, gas pressure or a cracked sensor insulator, so a technician confirms the cause. See HVAC error codes for how to read the light.
Flame-proving methods
| Method | Where used | Notes |
|---|---|---|
| Flame rod (rectification) | Most modern gas furnaces | Rod sits in the flame 1 |
| Remote flame sensing | Some furnaces | Sensor is on the far burner, proving flame carried across |
| Thermocouple | Older standing-pilot furnaces, water heaters | Heat makes a small voltage that holds the gas valve open |
| Flame sensor in a pilot assembly | Intermittent pilot systems | Proves the pilot before main burners open |
Example: reading the signal with real manufacturer numbers
Example only. These are the kinds of readings a technician takes with a microamp meter. The target comes from one Bosch furnace manual; other models differ 1.
| Reading | Value | What it suggests |
|---|---|---|
| Bosch target after cleaning | 1 to 6 µA at 115 V 1 | Normal range for that model |
| Technician reads before cleaning | Say 0.5 µA (illustrative) | Too low: coated sensor, poor ground, or misplaced rod |
| After a qualified tech cleans it | Say 3 µA (illustrative) | Back in range 1 |
| Trane board shows 8 red flashes | Code from the Trane IFC table 2 | "Low flame sense signal" |
The illustrative readings show the logic only. Measuring flame current means working live on a gas appliance with the burner compartment open, which is a technician's job.
Where you will see the term
Why it matters, and its weak points
What it protects
- Stops gas flow if the burner does not light.
- Shuts the gas off if the flame goes out mid-cycle.
- Gives a clear signal for diagnosis through blink codes.
Weak points
- Builds a coating over time that lowers the signal 1.
- Sensitive to poor grounding and wiring.
- Porcelain insulator can crack.
- Causes nuisance lockouts when dirty.
Why cleaning is not the whole fix
Bosch explains that "under some conditions the fuel or air supply can create a nearly invisible coating on the flame sensor," which acts as an insulator 1. Cleaning removes the coating, but if the cause is poor combustion, the coating returns.
Other causes look the same: a weak ground, reversed polarity on some boards, a cracked insulator, a misaligned rod, or flame lifting off the burners.
Trane's control even has a separate code for "115 volt AC power reversed or improper ground" 2. A repeat failure needs a full combustion and wiring check, not just another cleaning.
Sources: [1]
Cost drivers
HyreHVAC lists no repair prices without enough dated quotes. Drivers include whether a cleaning or a new sensor is needed, diagnosis time for repeat failures, related parts (igniter, wiring, board), and whether it is part of a maintenance visit.
Flame sensor service is often bundled into an annual heating check. Compare options in our maintenance plan comparison, and see repair drivers in the repair cost calculator.
What a technician does
Qualified technicians only, as the maker specifies 1.
- Read the board's fault code and the lockout history 1 2.
- Shut off power and gas before opening the burner compartment.
- Inspect the sensor, insulator and wiring; clean or replace the sensor.
- Check ground, polarity and burner flame appearance.
- Restore gas and power and measure flame current against the maker's range 1.
- Run several heating cycles to confirm stable operation.
What owners can do
- Book an annual heating check before the season; Bosch calls for yearly inspection by a qualified installer or service agency 1.
- ENERGY STAR's heating check-up includes gas connections, burner combustion and the heat exchanger 4.
- Keep the filter clean and the area around the furnace clear. Tune-ups pay off beyond the sensor: one study in DOE's review found a gas furnace tune-up with some duct improvements raised delivered capacity by 23% 5.
- Note the blink code (count the flashes) before calling, so the tech arrives prepared.
- Do not open the burner compartment yourself.
When to call a pro, and when to leave
- The furnace lights, then shuts off, again and again.
- The furnace is locked out and blinking a code.
- Burner flames look yellow, lazy or lift off the burners.
- A CO alarm sounds: get outside to fresh air immediately and call 911 3.
- You smell gas: leave the building, do not touch switches, and call the gas supplier from a neighbor's phone 1.
Safety standards and rules
Gas furnace ignition and flame-proving timing are set by national product safety standards for gas furnaces; Bosch's manual notes its control uses "ANSI standard rated module timing" 1. The control must stay as listed by the maker.
Installation and service follow the fuel gas code your area adopts, and work must be done by a qualified installer or service agency, per the manual 1.
CPSC recommends battery or battery-backup CO alarms on every level and outside sleeping areas 3. This page is held for review by a qualified reviewer before indexing.
Flame sensor vs other furnace safety parts
| Part | What it checks |
|---|---|
| Flame sensor | Flame is present after ignition |
| Limit switch | Heat exchanger is not overheating |
| Rollout switch | Flames are not escaping the burner area 1 |
| Pressure switch | Inducer is venting exhaust before ignition 2 |
| Igniter | Not a sensor; heats to light the gas |
Common misconceptions
- Myth A flame sensor problem is just a dirty rod.
- Reality Grounding, wiring, polarity and burner issues can cause the same failure 2.
- Myth If the furnace lights, the sensor is fine.
- Reality The sensor fails after lighting; the furnace then shuts the gas off.
- Myth It is a simple DIY job.
- Reality It is inside a gas burner compartment; the maker restricts it to technicians 1.
Climate and use
Furnaces in cold Northern states run more cycles each winter, so sensor coating and lockouts show up more there. Homes with gas furnaces anywhere benefit from a fall check before the first cold snap.
When it matters to you
It matters when your furnace lights and quits, locks out, or blinks a code. Count the flashes, check the code chart on the door, and call a technician.
Next step: see if it is urgent with the emergency checker, and read about what a heating inspection covers.
Questions about Furnace Flame Sensor
What does a furnace flame sensor do?
It proves that the burners are lit. After the gas valve opens, the control board looks for a small current through the flame to the sensor rod. If it does not see it, the board shuts off the gas to prevent unburned gas from building up.
What are the signs of a bad flame sensor?
The furnace lights, runs for a few seconds, then shuts off, often repeating before locking out. The control board may blink a code; on a Trane control, 8 red flashes mean low flame sense signal. Other faults can look similar, so a technician should confirm.
Can I clean my furnace flame sensor myself?
Manufacturers such as Bosch mark flame sensor cleaning for qualified service technicians only. It sits inside the burner compartment of a gas appliance. Book a technician, ideally as part of an annual heating check.
How long does a flame sensor last?
Many last for years with periodic cleaning, but coating builds up faster with poor combustion. The porcelain insulator can crack. A technician will clean or replace it based on its condition and signal reading.
What microamp reading should a flame sensor have?
It depends on the furnace. One Bosch manual lists 1 to 6 microamps at 115 volts after cleaning. Your furnace's manual gives its own range. Measuring it requires working on a live gas appliance, so it is a technician's task.
Why does my furnace keep shutting off after it lights?
A dirty or failing flame sensor is a common cause. Others include a poor ground, reversed wiring polarity, a cracked sensor, or burner problems. A technician reads the blink code and checks each.
Is a bad flame sensor dangerous?
The sensor's job is safety, so when it fails it usually shuts the furnace off rather than creating danger. The risk is losing heat. Any gas smell or CO alarm is a separate emergency: leave the building and call for help from outside.
What should I do if I smell gas near my furnace?
Do not light anything or touch electrical switches, and do not use a phone in the building. Leave immediately and call your gas supplier from a neighbor's phone. If you cannot reach them, call the fire department. That is the guidance in furnace manuals such as Bosch's.
How do I reset a furnace in lockout?
Many controls reset automatically after a period, or when power is cycled or the thermostat is turned off and back on, as the manual describes. If it locks out again, stop resetting it and call a technician. Repeated resets can hide a safety problem.
Sources
- Bosch, 96% AFUE Gas Furnace Installation, Operation and Maintenance Manual (04.2020), retrieved .
- Trane, Service Installer's Guide 18-CH111D1 (integrated furnace control and diagnostic codes), retrieved .
- U.S. CPSC, Carbon Monoxide Information Center, retrieved .
- ENERGY STAR, Maintenance Checklist (heating and cooling), retrieved .
- U.S. DOE Building Technologies Office, Residential HVAC Installation Practices: A Review of Research Findings (June 2018), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Sensor function, the 1–6 µA range, lockout count and gas-smell steps are quoted from Bosch's furnace manual; blink codes from Trane's installer guide, each labelled by brand. Held (G-REV) for a qualified reviewer. No DIY gas, combustion or electrical steps are given.
Suggest a correction. We fix errors and say what changed.