How to tell if a flame sensor is bad?

Author: Renee Collier  |  Last update: Wednesday, June 4, 2025

Now that you know why a flame sensor is necessary, consider these warning signs of a bad flame sensor in your furnace.
  1. Flue Line Corrosion. ...
  2. High Energy Bills. ...
  3. Yellow Flames. ...
  4. Furnace Short Cycling. ...
  5. Dirt on the Sensor. ...
  6. Visible Damage on the Flame Sensor. ...
  7. Age.

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.

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.

Will a furnace ignite with a bad flame sensor?

If it's broken or malfunctioning, it may fail to detect the flame and shut off the gas supply as a precaution, even if the burners are actually lit. This can result in your furnace starting the ignition process but failing to stay lit.

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 To Test A Furnace Flame Sensor

How often do furnace flame sensors go bad?

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.

What causes a flame sensor to go out?

Do flame sensors wear out? Flame sensors wear over time. However, often the problem is a dirty sensor, rather than a broken one. Since these sensors are designed to have a low tolerance for reading variations, they might misread from the slightest carbon coating.

Why is my furnace turning on but not igniting?

Defective Igniter or Pilot Light

If it clicks but your furnace won't ignite after the combustion cycle, the electronic ignition systems probably need to be replaced. In cases where the furnace ignition starts normally but only stays on for a few minutes and then goes out, a new pilot light likely needs to be installed.

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.

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.

How do I know when to replace my flame sensor?

Here are common indicators that suggest your furnace's flame sensor may be bad:
  1. Furnace Burners Turn On and Then Go Out After a Few Seconds. ...
  2. Yellow Burner Flames. ...
  3. Rusted Flue Lines. ...
  4. High Utility Bills. ...
  5. Old Age. ...
  6. Exhaust Smells. ...
  7. Constant Thermostat Adjustments. ...
  8. Short Cycling.

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 do you test a flame sensor to the ground?

Attach one wire from your multimeter to the flame sensor wire and the other one to the unit where you unplugged the wire. Turn on the unit. Look for a reading between . 5 and 10 micro-amps with the normal range between 2 and 6.

How do I force my furnace to start?

Checking your thermostat is the best place to start. Make sure it's set to “heat” and that you've set the temperature higher than the current room temperature. If that doesn't resolve the problem, try switching your fan to “on” to force the furnace to start working.

Can hear your furnace click on but it does not fire up?

Most likely, your furnace is making a clicking noise and not providing heat because your furnace's spark igniter is trying to ignite your pilot light or burners but is failing to do so. In other words, there's an ignition system malfunction that needs to be fixed. Otherwise, your furnace won't start.

Can you manually light a gas furnace with electronic ignition?

Like an extinguished pilot, a faulty ignitor prevents a furnace from heating up. Unlike pilots, electronic ignitors cannot be manually lit when they fail. However, a homeowner can take troubleshooting measures to determine if the ignitor is bad or if some other issue is causing the furnace not to blow hot air.

What are the symptoms of a faulty flame sensor?

As homeowners, we should be aware of these signs:
  • Furnace short cycling, where the burner ignites but quickly shuts off.
  • Inconsistent heating throughout our home.
  • Repeated safety ignition lockouts after three failed attempts.
  • Burner flames or pilot lights that extinguish shortly after ignition.

Will a flame sensor cause a furnace not to ignite?

If your furnace won't ignite, the source of the issue may be a broken flame sensor. The sensor is responsible for turning off the gas burners to avoid gas buildup. Once this component starts to malfunction, it may stop the burners from igniting altogether.

How much does it cost to replace a flame sensor on a furnace?

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.

Should I clean or replace a flame sensor?

It is not likely a flame sensor needs to be replaced. You should be able to sufficiently clean a sensor no matter how dirty it is. But they are not expensive parts, and you can choose to replace it instead of cleaning it. There are different shapes of flame sensors so be sure you are purchasing the proper kind.

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