If your furnace lights for a few seconds and then shuts off, or it won't start at all after a few tries, it has probably locked itself out for safety. Here's how to reset it, and how to fix the most common cause.
How to Reset a Furnace
- Turn the thermostat off or down so it isn't calling for heat.
- Turn off power at the furnace switch. It looks like a light switch on or near the furnace. Wait 30 seconds.
- Turn the power back on and set the thermostat to call for heat again.
- Watch the startup. The inducer fan should run first, then the igniter glows or sparks, then the burners light.
Some older furnaces also have a red reset button on the blower motor. Press it once. If the furnace trips again, don't keep pressing it. Repeated resets can overheat the motor or hide a real safety problem.
Signs of a Dirty Flame Sensor
The flame sensor is a thin metal rod that sits in the burner flame. It proves to the control board that the gas actually lit. When a film of carbon builds up on it, the board can't sense the flame and shuts the gas off as a precaution. The classic pattern:
- The burners light, run for 3 to 10 seconds, then shut off.
- The furnace tries again two or three times.
- Then it locks out and a light on the board blinks a code.
How to Clean the Flame Sensor
If you're comfortable working inside the furnace, this is a common homeowner fix. If not, it's a quick part of every tune-up.
- Turn off the power at the furnace switch and turn the gas valve to off.
- Remove the burner access panel and find the flame sensor. It's a single metal rod with a white ceramic base, held in by one small screw, sitting in front of a burner.
- Remove the screw and gently pull the sensor out. Don't touch the rod with your fingers. Skin oils make it dirty again faster.
- Lightly clean the rod with fine steel wool or a very fine abrasive pad until it's shiny. Don't use coarse sandpaper.
- Reinstall the sensor, replace the panel, turn the gas and power back on, and test.
When to Call a Technician
- The furnace still locks out after a reset and a clean sensor.
- The light code points to a pressure switch, igniter or limit switch.
- You smell gas, see a yellow flame, or your CO alarm sounds. Leave the house and call the gas company from outside.
A flame sensor that keeps getting dirty fast can be a sign of a combustion problem, so it's worth having it looked at during your yearly furnace tune-up.
