Furnace clicks but won't ignite: the 5-minute flame-sensor fix
⏱ 5 min read · 💲 $0 · Updated 2026-09-27 ·
The most common furnace no-heat call is a dirty flame sensor. It takes a piece of fine sandpaper and five minutes.
Symptom
Furnace starts, igniter glows or clicks, burners light for 3–5 seconds, then shut off. Repeats 3 times, then locks out.
Fix
- Turn off power at the switch on the furnace and close the gas valve.
- Remove the burner compartment door.
- Find the flame sensor: a thin metal rod on a ceramic base with one wire, positioned in the flame path opposite the igniter.
- Remove the single screw, slide it out, and gently polish the rod with fine (400+) sandpaper or steel wool. Wipe clean.
- Reinstall, restore gas and power, and test.
Stop and call a pro if: you smell gas, the igniter never glows, or the burner flames are yellow/orange instead of blue. Cracked heat exchangers and gas-valve faults are not DIY.
Other quick checks
- Thermostat set to Heat and calling above room temperature
- Filter not clogged (a blocked filter trips the high-limit switch)
- Condensate drain not blocked (high-efficiency units)
- Exhaust and intake pipes clear of snow, nests or debris
Still no heat? Service calls run $80–$250 and a furnace repair typically $150–$1,200. Get 3 quotes →
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