How To Replace A Gas Furnace's Flame Sensor

A gas furnace has a flame sensor that detects that the pilot light is in fact lit when there is gas coming into the system. If the sensor doesn't sense the light, the gas supply is shut off. A dirty or malfunctioning flame sensor can make the gas supply shut off even when there is a pilot light present.

If your heater has stopped putting out heat even though you can see the pilot light, you might have a dirty or broken flame sensor. Cleaning or replacing the flame sensor is a do-it-yourself project suitable for those with moderate home improvement experience.

What You Need:

  • Screwdriver
  • Socket wrench
  • Steel wool
  • New flame sensor, if needed

Step 1: Access the Flame Sensor

Turn off the electrical supply to the furnace at the circuit breaker. Twist the shut-off valve on the gas supply line in a counterclockwise direction until you can't twist it any further.

Locate the upper access panel on the furnace and remove its fasteners, which should either be screws, levers, or twistable knobs. Pull off the panel door and set it aside so you don't trip while you work.

Locate the flame sensor on the burner assembly. Use a socket wrench to remove the fastener under the flame sensor. Examine the tip of the sensor for any signs of dirt or buildup. If the sensor looks dirty, proceed to Step 2. If the sensor looks damaged, proceed to Step 3.

Step 2: Clean the Flame Sensor

Clean the end of the flame sensor with a piece of steel wool. Reattach the mounting screw and the access panel door with its fasteners. Restore gas and electric supplies to the unit.

Perform a test cycle of your heater to see if the flame sensor is now working. If the sensor still seems to have issues, you can follow Step 1 to take the unit back apart and then proceed to installing a new flame sensor.

Step: Install New Flame Sensor

Pull out the old flame sensor and you will notice a wire attached to the rear end. Find the connection point between the flame sensor's wire and that of the furnace and pull the wires apart. Discard the old flame sensor.

Take the new flame sensor out of its packaging. Connect the dangling furnace wire to the wire on the flame sensor. Place the rear end of the flame sensor into the hole where the old sensor was located. Replace the mounting screw and twist it into place with the socket wrench.

Put the upper access panel back on and tighten its fasteners. Turn on the gas supply valve, moving in a clockwise direction until the handle stops. Restore the electricity to the unit.

Use your thermostat to start up the heater and wait to see what happens. If your unit still isn't heating or heating efficiently, the problem runs deeper than the flame sensor alone and you should call in an HVAC repair company. Find out more by contacting an HVAC business.