A flame sensor is a small metal rod that sits in the burner flame on a gas furnace. Its job is to confirm that the burner has actually ignited — if no flame is detected within a few seconds of the gas valve opening, the control board shuts off the gas to prevent dangerous unburned-gas buildup.
How they fail
Flame sensors don’t typically “break” mechanically — they get coated with carbon, oxidation, or burner residue. The coating blocks the sensor’s ability to detect the flame’s ionization signal. The furnace then thinks there’s no flame, shuts down the gas, and tries again — over and over — until it locks out.
Symptoms of a failing flame sensor
- Furnace ignites, runs 30-90 seconds, then shuts off
- Cycles repeatedly trying to relight
- Eventually goes into hard lockout (LED flashing fault codes)
- No heat despite normal-sounding startup
The fix is simple and cheap
Cleaning or replacing a flame sensor is one of the fastest HVAC repairs — typically 20-30 minutes. Most repairs run $165-$225 including the trip. Many techs simply clean the existing sensor with light steel wool; we replace them on units 10+ years old where the sensor is approaching end-of-life.
Related: Furnace, Short Cycling, Inducer Motor.
Keep Reading
Right Way Mechanical is a family-owned HVAC contractor serving Wheat Ridge, Arvada, west Denver, and the Colorado foothills.