How to Test a Furnace Flame Sensor (Flame Rod)

Published: July 23, 2026 — BC Wide Home Services Ltd, doing business as BC Wide Heating & Air Conditioning — Greater Vancouver, BC

Testing the Flame Sensor

The flame sensor generates microamps (µA) when the flame is present. To test: turn off power, disconnect the wire from the sensor, connect your multimeter in series (set to DC microamps), turn the furnace on, and read the current. A good sensor reads 2-10 µA. Below 1.5 µA: weak signal (clean or replace). 0 µA: sensor or wiring fault.

Cleaning the Sensor

If the sensor has carbon buildup, cleaning often restores function. Remove the sensor (one screw), gently clean the metal rod with fine steel wool or a scotch-brite pad — do not use sandpaper (too abrasive). Wipe clean with a dry cloth. Reinstall and test. Cleaning resolves 90% of flame sensor issues.

Replacement Cost

Flame sensor replacement: $20-50 part, $150-250 installed (including cleaning attempt). Most sensors are universal and inexpensive. Annual cleaning during furnace maintenance prevents flame sensor issues. If your furnace short-cycles (fires, runs for 2-5 seconds, then shuts off), the flame sensor is the most likely cause.

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Safety Reminder

If you smell gas, suspect carbon monoxide or believe there is an immediate danger, leave the property and contact emergency services or the appropriate gas emergency authority. Do not remain inside — exit the building immediately and call for help from outside.