Bad flame sensor: symptoms, cleaning, replacement

The flame sensor is a safety device. Once the gas valve opens, the control board needs proof that the gas is burning. The sensor rod sits in the flame from one burner, and the flame itself carries a tiny electrical current from the rod to the grounded burner. The board reads that current. If the reading is too weak, the board assumes there is no flame and closes the gas valve so unburned gas cannot build up.

Over time a thin layer of oxide and combustion residue coats the rod and insulates it. The flame is there, and the board can no longer see it.

• Burners light normally, then shut off after a few seconds.
• The furnace tries several times in a row, then stops and stays off for a long while.
• The blower runs and the air from the registers is cool.
• The control board flashes a code for flame loss or ignition lockout.
• The problem comes and goes at first and gets more frequent.

The same pattern can look like furnace short cycling. The difference is timing. A flame sensor fault shuts the burners off within seconds of lighting. Overheating shuts them off after several minutes of running.

Cleaning is a short job for a technician. With the power off, the sensor is unscrewed from the burner assembly, the rod is wiped gently with a fine abrasive pad to remove the coating, and the porcelain insulator is checked for cracks. The sensor goes back in the same position and the technician measures the flame signal with a meter to confirm it is strong and steady.

That last step matters. Without a meter reading, nobody knows whether cleaning fixed the problem or only bought a few days. Rough sanding also scratches the rod and makes it foul faster.

This site does not walk you through doing it yourself. The sensor sits beside gas and electrical parts, and the furnace door is there to keep hands out of that space.

A clean sensor can still read low. Other causes a technician will check:

• A cracked insulator or a loose or corroded sensor wire.
• A poor ground connection between the burners, the furnace cabinet and the control board.
• Dirty burners or low gas pressure giving a weak, lifting flame that does not reach the rod.
• A failing control board that misreads a good signal.

If the rod is pitted, bent or the insulator is cracked, the sensor is replaced. It is one of the smaller parts on a furnace, and the labor is brief once the diagnosis is confirmed. The ignitor and flame sensor cost guide covers what goes into the price.

A flame sensor fault is never a reason to replace a furnace. If it fouls again within one season, ask the technician to look for the cause, such as dirty burners or poor combustion air. The furnace repair guide lists the other common faults.