Reviewed by Al, the Building Doctor.
IUOE Local 39 Stationary Engineer (commercial gas heating)
30 years facilities — winter no-heat calls + flame sensor diagnostics
Power off at the furnace switch FIRST
Light-switch on the side of the furnace, OFF. Smell gas? Get out of the house first, call the utility. Don't proceed if there's any gas odor.
The 6-step clean
- Cut power at the furnace switch. Verify by trying to start — should not start.
- Remove burner door. Slides up and off (most). Some have screws.
- Locate the flame sensor. Single ceramic rod with one wire, sitting in the burner flame path. Bent at 90°, ~3 inches long, white ceramic insulator at the base. Don't confuse with the hot surface igniter (HSI) — the igniter is also ceramic but has two wires and is positioned differently.
- Remove the sensor. Single sheet-metal screw (1/4" hex or Phillips). Unscrew. Pull the sensor straight out. Disconnect the spade-lug wire.
- Polish the rod. Fine emery cloth (400-grit) or a folded dollar bill. Rub gently until the rod is shiny silver. Don't use steel wool — leaves conductive abrasive residue that fouls the rod faster. Don't use a wire brush — too aggressive.
- Reinstall + restore power. Reconnect wire, slide rod back into the burner channel (don't bend it), tighten screw. Replace burner door. Restore power. Cycle the thermostat. Should fire and stay lit.
When cleaning doesn't fix it
- You've cleaned it twice in a season. Replace the rod ($15-$25 universal — Honeywell or White-Rodgers). The carbon-recurrence pattern means the rod itself is going.
- Symptom persists with a new rod. Burner ground issue (check the green ground wire from the burner assembly to the cabinet — corroded or loose?), cracked porcelain insulator at the base, or control board problem.
- You get a different flash code after cleaning. Different problem now exposed. See furnace won't ignite diagnostic for the brand-by-brand flash codes.
FAQ
How do I know if my flame sensor is dirty?
Furnace lights, runs 3-5 sec, gas shuts off. 2-3 retries then lockout. 60-80% of winter no-heat calls.
Can I use sandpaper or steel wool?
No to steel wool. Fine 400-grit sandpaper or folded dollar bill. Don't use wire brush.
How often clean the flame sensor?
Annually preventive. If cleaning twice in a season, replace it ($15-$25).
What if cleaning doesn't fix it?
Replace rod ($15-$25). Still no fix: burner ground issue, cracked porcelain, or control board.
Related guides
- Furnace won't ignite — the diagnostic that points here
- Best universal furnace igniters 2026 — the adjacent wear part
- Furnace banging when starting — separate symptom, often confused
Editorial standards: Reviewed by Al, Building Doctor — IUOE Local 39 Stationary Engineer with commercial gas-heating diagnostic experience.