How do I know when to replace my flame sensor?

Author: Collin Schaefer  |  Last update: Monday, June 30, 2025

3 Symptoms of a Bad Flame Sensor & What to Do
  1. Your Furnace Turns On and then Off Quickly. If your furnace starts up, but then turns off within a few seconds it is short cycling. ...
  2. The Flame Sensor has a Crack. The outer part of the flame sensor is porcelain. ...
  3. You Can See Dirt or Corrosion on the Sensor.

What are the symptoms of a bad flame sensor?

The sensor's failure to detect a flame causes the furnace to shut down as a safety measure. Inconsistent heating: A faulty flame sensor can cause the furnace to cycle on and off frequently, leading to inconsistent heating in your home. You may notice hot and cold spots or a lack of comfortable warmth.

How often should a flame sensor be replaced?

Having a working flame sensor is critical for a functioning furnace. While most furnaces last for 10 to 20 years, flame sensors have a much shorter lifespan of about five years. Usually, when a flame sensor goes bad, the best route is to replace it outright.

What is the life expectancy of a flame sensor?

While the average life span of a flame sensor is about five years, many technicians recommend replacing the flame sensor a little more often at every two to three years. This schedule helps ensure that the flame sensor is always in good condition and not causing problems with the gas supply.

Should I clean or replace a flame sensor?

Most of the time, the flame sensor only needs to be cleaned, not replaced. It is little more than a stainless steel rod that pokes into the flame, and carries a tiny electrical current to sense the presence of the flame. This is different than the thermocouples used with a standing pilot system.

How To Check A Flame Sensor - Troubleshooting 101 - Furnace will not stay lit

How do I know if I need a new flame sensor?

Your Furnace Turns On and then Off Quickly

If your furnace starts up, but then turns off within a few seconds it is short cycling. Short cycling is a common sign of a bad flame sensor.

What will a dirty flame sensor cause?

The flame sensor can become corroded due to carbon buildup from the flame. This can cause the sensor to fail. If the burner light goes out within seconds of switching on the unit, this is a telltale sign of a dirty sensor. If you see soot clearly covering the sensor, then it's time for a cleaning.

How much does it cost to replace a flame sensor?

Most furnace flame sensors cost less than $20, but if you opt to have a professional HVAC technician make the repair, you can expect to pay between $75 and $250. Need help replacing your flame sensor?

Is a flame sensor hard to replace?

Similar to a thermocouple, replacing a flame sensor can be a hard task and should usually be left to the pros.

What is the difference between a smoke sensor and a flame sensor?

Flame detectors are also effective in detecting fires in areas where smoke detectors would be prone to false alarms, such as kitchens, garages, and other areas with high levels of smoke or steam. Flame detectors are also less prone to false alarms than smoke detectors.

How do you reset a flame sensor?

There is a work-around to waiting. If there is no safety risk (like a gas leak), a furnace can be reset to bypass this wait. Turn off the furnace's power switch and wait 10 seconds before turning it back on.

Do gas furnace flame sensors go bad?

A furnace flame sensor can fail due to several reasons, including: Dirt buildup that prevents it from detecting the flame. Corrosion from moisture or improper maintenance.

Can I run my furnace without a flame sensor?

1. Will a furnace run without a flame sensor? No, a furnace will not operate without a flame sensor.

How long does it take to replace a flame sensor?

Flame sensors have an average lifespan of about five years. Therefore, you can expect to need to replace it at least twice before replacing your entire heating system.

Is a flame sensor and thermocouple the same thing?

People tend to use the terms “flame sensor” and “thermocouple” interchangeably. In truth, while they basically perform the same job, they're not the same thing. If your furnace uses an electronic controller, it's got a flame sensor. Thermocouples are used on standing pilot furnaces that have pilot lights.

How much is a typical current flame sensor current?

When the flame lights, you should read between 0.5 and 10 microamps (μA), depending on the furnace. Readings between 2 and 6 are common.

Does cleaning a flame sensor work?

If you suspect the flame sensor is dirty or malfunctioning, cleaning it might help restore proper function. However, if you're unsure or uncomfortable performing maintenance yourself, it's a good idea to call a professional HVAC technician for a thorough inspection and service.

Why is my gas heater blowing cold air?

Pilot Lights and Flame Sensors Could Be the Issue

A modern furnace without a pilot light relies on flame detectors to ensure the successful completion of a heating cycle. If the flame detector is covered in dust, dirt, or grime, you will quickly lose heat and your gas furnace will start blowing cold air.

What is the most expensive part to fix on a furnace?

A heat exchanger replacement is often one of the most costly repairs in a furnace, and the expense can sometimes rival that of installing a new furnace. Typical replacement costs vary widely but can easily reach several thousand dollars, especially if labor costs are factored in.

Why does my furnace keep lighting and going out?

If your furnace keeps turning off and on, your thermostat may be the culprit. A malfunctioning or improperly placed thermostat can cause your furnace to short cycle. Common issues include: Old wiring or dead batteries: Replace batteries or old wiring to ensure accurate temperature readings.

Are all furnace flame sensors the same?

While all flame sensors perform the same job, they may operate or work using different properties. For example, some sensors use ultraviolet detection, while others use infrared detection to sense the flame. Occasionally, furnaces have flame sensors that feature both infrared and ultraviolet.

What is a limit on a furnace?

A furnace limit switch is a component of a forced-air furnace that is responsible for turning on the furnace blower once the desired heat has been met inside the furnace. Without a functioning furnace limit switch, a furnace would not be able to regulate the temperature in conjunction with a thermostat.

Why wont the pilot light stay lit on my furnace?

Dirty or Damaged Thermocouple Explains Why Pilot Light Keep Going Out. A faulty, misaligned, dirty, or damaged thermocouple is often the culprit behind your pilot light going out. The thermocouple is an automatic safety shutoff device that prevents the gas valve from opening if the pilot light is out.

Is there a coating on a flame sensor?

Most flame sensors have a coating on them to keep them clean; with this in mind, you should avoid using anything too abrasive, like heavy grit sand paper. This will wear down the coating and cause the sensor to get dirty at a more rapid rate in the future.

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