Clean or Replace a Flame Sensor (Furnace)

Decide clean-vs-replace, make sure the rod sits IN the flame, lightly clean it (don't grind/sand the rod down), and verify the microamp flame-rectification signal in µA series — typically ~2–6 µA, with lockout below ~0.5–1 µA.

Approx. 30 min

Before you start

•Symptom matches a weak flame signal: burners light then drop out after a few seconds (flame-loss lockout / short-cycling on ignition), confirmed by a low/zero µA reading.
•Decide clean vs replace: a sooted/oxidized but straight rod cleans up; a cracked porcelain insulator, a burned-through rod, or one that won't hold signal after cleaning gets replaced with the OEM sensor.
•Power can be killed at the furnace switch before pulling the sensor.

Tools & materials

Multimeter capable of microamps (µA DC) in series · Light abrasive pad / fine steel wool (NOT coarse sandpaper) · Nut driver / screwdriver · Replacement OEM flame sensor (if replacing)

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Remove & inspect

Kill power and pull the sensor
Turn off the furnace switch, remove the single mounting screw, and withdraw the sensor rod with its lead. Inspect the porcelain insulator for cracks (a hairline crack shorts the signal to ground).
⚠ Power off before removal; the rod runs through a live burner flame area.
Confirm the rod sits IN the flame
Check the rod tip is engulfed by the burner flame when lit — a sensor that's too high, bent away, or carboned won't rectify enough current. The flame must bridge the rod to the grounded burner.
Target: Rod tip fully in the flame, with a solid burner ground path

Clean or replace

Light-clean the rod only
Wipe the rod with a fine abrasive pad or fine steel wool to remove oxide/soot — just brighten it. Do NOT grind it down with coarse sandpaper; you remove material and roughen the surface, which lowers the signal long-term.
Target: Rod brightened, not reduced; no coarse abrasive
⚠ Light cleaning only — grinding the rod shortens its life and degrades the µA signal.
Replace if cracked or weak
If the insulator is cracked, the rod is burned thin, or the signal won't recover after cleaning, install the OEM sensor in the same position with a good lead connection. Don't substitute a mismatched rod length.
Target: OEM sensor, same position/length; clean tight lead

Verify the µA signal

Measure flame rectification in microamps
Put the meter in series on the flame-sensor lead (µA DC) and run a heat call. A healthy signal is typically ~2–6 µA. Below roughly 0.5–1 µA the board will see no flame and lock out.
Tools: Multimeter (µA series)
Target: Flame signal ~2–6 µA typical; <~0.5–1 µA = lockout territory
Run full cycles to confirm it holds
Restore normal wiring and run several cycles. The burners should light and stay lit with a stable µA signal — a signal that decays over a cycle means a marginal rod, a poor ground, or a flame that's pulling off the rod.
Target: Stable signal across multiple cycles; no flame-loss dropout