Cleaning a furnace flame sensor is usually a quick DIY job, but it must be done with the furnace fully powered down and only if you can access the part safely.

Safety first

  • Turn off the furnace at the thermostat and set it to “Off.”
  • Shut off electrical power to the furnace at the service switch or breaker, and close the gas shutoff valve if accessible and labeled.
  • If you smell gas, see scorch marks, or feel unsure at any point, stop and call a licensed HVAC technician rather than continuing.

Find the flame sensor

  • Remove the burner/access panel on the front of the furnace; it typically lifts off or is held by a few screws.
  • Look near the burner assembly for a thin metal rod (about 3–4 inches) with a single wire attached and a small mounting screw, often set in a white porcelain insulator.
  • Do not confuse it with the igniter, which is often on the first burner, while the flame sensor is commonly on the last burner in the row.

Remove the sensor

  • Use the appropriate screwdriver to remove the single mounting screw holding the sensor bracket.
  • Gently slide or pull the sensor out of the burner area, then carefully disconnect the wire connector, noting how it was attached for reassembly.
  • Inspect the porcelain for cracks and the rod for heavy corrosion; if cracked or badly pitted, replacement is usually recommended instead of just cleaning.

Clean the flame sensor

  • Use fine‑grit sandpaper or emery cloth (around 220 grit or finer) to lightly polish the metal rod, removing soot and carbon until it looks clean and slightly shiny.
  • Avoid aggressive abrasives like coarse sandpaper or heavy steel wool that can scratch and shorten the life of the sensor.
  • Wipe the rod with a clean, dry cloth, or use a cotton swab with isopropyl alcohol to remove any remaining residue, then dry thoroughly.

Reinstall and test

  • Reconnect the sensor wire firmly, then slide the sensor back into position so it sits in the burner flame path, and reinstall the mounting screw snugly (not over‑tightened).
  • Replace the access panel, reopen the gas valve, turn the breaker/switch back on, and then set the thermostat to “Heat” to call for heat.
  • Watch at least one full cycle: the burners should light, stay on steadily, and the furnace should run without rapid on‑off short cycling; if it still shuts down, call a professional, as the issue could be wiring, control board, gas pressure, or a failing sensor that needs replacement.

Mini TL;DR:
Turn off power and gas, remove the access panel, gently pull and disconnect the thin‑rod flame sensor, lightly clean the rod with fine sandpaper or alcohol until shiny, wipe it, reinstall, restore power and gas, and test a full heat cycle.

Information gathered from public forums or data available on the internet and portrayed here.