Flame Sensor Cleaning, Inspection, and Ohm Test Step by Step Guide
Flame Sensor Cleaning, Inspection, and Ohm Test
Safety first: shut off all power to the furnace at the service disconnect or breaker, shut off gas supply, verify zero volts with a meter, and follow lockout/tagout procedures before you begin.
Tools and materials
- Multimeter with continuity and low-resistance ranges
- Steel Wool 0000 Or metal wire brush
- Soft brush and vacuum or canned air
- Nut driver / screwdriver set to remove sensor mounting screw
- Small mirror or flashlight for inspection
- Gloves and safety glasses
Step 1 — Access and remove the flame sensor
1. Turn off power and gas to the appliance and verify with meter.
2. Remove furnace access panel to expose burner assembly and sensor.
3. Note sensor routing and wiring; photograph if needed.
4. Disconnect the single lead from the sensor (pull the female spade off; do not lift by the wire).
5. Remove the sensor mounting screw and carefully withdraw the sensor from the burner tube.
Step 2 — Visual inspection
- Check the sensor rod for heavy carbon, soot, pitting, corrosion, bending, or burned/insulated wiring at the connector.
- Inspect the mounting hole for loose screws, warped bracket, or evidence of arcing.
- If the rod is physically damaged, replace it.
Step 3 — Clean the sensor
1. Gently wipe loose dust with a soft brush and vacuum.
2. Lightly polish the stainless rod with fine 0000 steel wool until the surface is bright.
3. DO NOT USE SANNDPAPER as the glue can stick to the flame sensor causing problems. Use steel wool or steel brush.
4. Remove all polishing debris and wipe clean with a lint‑free cloth or canned air.
5. Avoid aggressive sanding that changes rod diameter or shape; do not coat the rod with cleaners or oils.
Step 4 — Reinstall and tighten
- Reinsert the sensor into its original hole so the sensing tip extends the same distance into the flame area as before.
- Fasten the mounting screw snugly; avoid overtightening which can bend the rod.
- Reattach the lead to the spade terminal ensuring a solid, corrosion‑free connection.
- Replace access panel but keep gas/power off until ready to test.
Step 5 — Ohm/continuity check (what it tells you)
- Purpose: a basic continuity check can show an open circuit or gross wiring damage but does not confirm flame rectification function.
- Meter setup: set multimeter to continuity or the lowest ohms range.
- Probe points: one probe to the sensor rod (exposed metal) and the other probe to the sensor lead terminal while it is disconnected from the control board.
- Expected reading: most intact flame sensors will read a very low resistance or near short when measured end‑to‑end through the sensor body and connector; many reading results vary by model so treat this as a verification that the sensor is not open.
- Interpretation:
- Reads OL or infinite resistance: sensor is open or wiring broken, replace sensor.
- Reads low resistance/continuity: sensor and wiring are likely intact electrically but this does NOT guarantee proper flame sensing operation.
Step 6 — Recommended functional test (flame rectification test)
- Why: the correct diagnostic for a flame sensor is measuring the flame current or verifying the control’s flame-detection circuit, not just ohms.
- Simple functional check: with power and gas restored, observe that the furnace control gets a steady flame signal after ignition (control lights or diagnostic LED will show flame detected).
- Professional test: use a flame current meter or multimeter in microamp DC mode placed between the sensor lead and ground to measure the microamp-level rectified current; typical flame currents are in the range of 2–10 μA depending on system; consult the appliance manual for expected values.
- If flame current is below spec despite a clean sensor, check grounding, burner flame coverage on the sensor, sensor tip position, control board input, and possible electrical leakage or poor chassis ground.
Common causes of sensing failure
- Carbon/soot coating on sensor rod
- Sensor positioned too far from or out of the flame envelope
- Poor ground/chassis connection or control board corrosion
- Intermittent connector or frayed wire
- Damaged or pitted sensor rod
- Low flame quality or burner misalignment reducing rectification current
Quick troubleshooting flow
- Clean sensor → reinstall correctly → continuity check → run furnace and observe control LED/operation → if still failing, measure flame current in μA → if low, check ground, flame shape, and control board input → replace sensor only after cleaning and verifying installation.
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· By: Darin DeVries
· HVAC Part Store LLC
· Coleman HVAC Parts
· Technical Service Advisor
· Author, entrepreneur, e-Commerce