Why does my furnace light then shut off?
Updated August 2026
A furnace burner that ignites, holds for a few seconds, then goes dark is the textbook sign of a dirty flame sensor that can no longer prove the flame to the control board. The burner lights for 3–7 seconds, the board never sees a flame signal, so it closes the gas valve as a safety. Cleaning the sensor rod is free at home or $80–200 for a tech, and it is the first thing to rule out.
Safety first: cleaning the flame sensor is a power-off, gas-off job: shut the furnace switch and the gas cock before pulling the rod. Never jumper or bypass the flame sensor to keep the burner lit, because that defeats the safety that shuts the gas off when the flame fails. Keep carbon-monoxide detectors active on every floor while you work, and if you smell gas, leave and call the utility.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Dirty flame sensor | Most common | The burner lights 3–7 seconds then dies because the sensor cannot prove flame. Scuff the rod clean or have a tech do it. | Free DIY to $200 |
| Weak or cracked hot-surface igniter | Common | A failing igniter glows but lights the gas late or weakly. Replace the igniter. | $150–400 |
| Poor burner ground | Occasional | The flame-sensing circuit needs a solid ground. A corroded connection mimics a dirty sensor. | $80–200 |
| Failing gas valve | Occasional | A weak valve delivers unstable gas and the flame drops. A tech tests and replaces it. | $300–600 |
| Clogged condensate drain trips pressure switch | Occasional | A blocked trap on a high-efficiency furnace opens the pressure switch mid-cycle. Clear the trap. | $0–200 |
Check these first
- Count the seconds. A burner that holds 3–7 seconds then drops out points almost dead-on at the flame sensor.
- Shut the power and gas off, pull the flame sensor rod, and gently scuff it clean with a fine abrasive pad or a folded dollar bill.
- Check the igniter through the sight glass. It should glow bright orange, not dim, spotty, or cracked.
- Look for a clogged condensate trap on a condensing furnace, which can trip the pressure switch mid-cycle.
- Note whether it locks out after three tries; many boards flash an error code you can read against the door legend.
Prevent it next time
A flame sensor fouls with a normal film of carbon and silica, which is why the fix is maintenance, not a new part. Have the sensor and burners cleaned at the yearly furnace test before winter, and change the filter every 30–90 days so the burner runs clean. A $100–200 tune-up prevents the mid-January no-heat call this symptom becomes, and it is when a tech would flag a tiring igniter before it strands you.
When to call a pro
If cleaning the flame sensor does not hold the flame, hand it off: the next suspects (igniter, gas valve, control board, pressure switch) need meters and gas-safe handling. Expect $80–200 for a sensor clean, $150–400 for an igniter, and $300–600 for a gas valve. Any smell of gas, or a repeating lockout with soot around the burner, is a call-now situation, and a furnace past 15 years may be cheaper to replace than to keep patching.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How do I clean a furnace flame sensor?
Turn off the furnace switch and the gas, then find the flame sensor: a thin metal rod in the burner flame path with a single wire. Remove its one screw, slide it out, and gently scuff the metal with fine steel wool, an emery cloth, or a folded dollar bill until it is dull-clean. Do not use coarse sandpaper, which scratches it. Reinstall, restore power and gas, and it should hold the flame. Total cost: nothing.
How often does a flame sensor need cleaning?
About once a year for most furnaces, which is exactly why it belongs in a fall tune-up. The rod slowly builds a film of carbon and silica from normal combustion until it can no longer conduct the tiny current that proves flame. Homes with more dust, or a furnace that runs hard, may need it twice a season. If cleaning fixes the drop-out but it returns within weeks, the sensor is worn out and a $15–30 replacement rod is due.
What is the difference between a flame sensor and an igniter?
They sit side by side but do opposite jobs. The hot-surface igniter glows orange to light the gas at startup, then powers off. The flame sensor is a separate rod that stays in the flame and proves it is burning, telling the board to hold the gas valve open. If the burner never lights, suspect the igniter ($150–400 to replace); if it lights then dies after a few seconds, suspect the flame sensor (free to clean).