Will a furnace ignite with a bad flame sensor?

Author: John Ankunding  |  Last update: Wednesday, August 6, 2025

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.

Will a flame sensor cause a furnace not to ignite?

If the flame sensor can't sense a flame, it will turn the gas valve off (and the furnace off) so that you are not releasing gas that is not being burned away. To see what might be the issue with your furnace, first switch off the power to your furnace and then take off the panel cover to see what's going on inside.

How to tell if a flame sensor is bad on a furnace?

Top 7 Signs of a Bad Furnace Flame Sensor
  1. Flue Line Corrosion. The flue line is responsible for venting harmful gases from the furnace outdoors so nothing seeps into the house. ...
  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.

Can I run my furnace without a flame sensor?

You can't bypass a flame sensor. The circuit board in a furnace will sense the flame (yes a flame has a slight amount of conductivity) and will not turn on the main burner if its not present. It would be very dangerous if you could do it.

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.

Furnace Lights Then Goes Out - Symptoms of Dirty or Bad Flame Sensor

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.

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.

Will a furnace fire with a bad flame sensor?

Without a flame sensor, your furnace will keep emitting gas even when there's nothing to ignite it, resulting in hazardous gas buildup. If your flame sensor is dirty or faulty, it will automatically shut down the heating unit if it doesn't detect a flame.

Is a furnace flame sensor or ignitor?

For instance, the flame sensor is on the left side, while the ignitor is located on the right side of the burners. When the burners produce a flame, the rod-shaped protrusion detects it and sends an electrical signal to the furnace's control board, signifying that everything is functioning correctly.

How to reset furnace 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.

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.

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?

What are the signs of a bad flame sensor on a furnace?

Recognize common symptoms of flame sensor failure: short cycling, frequent shutdowns, and specific error codes on the furnace display. Clean the flame sensor regularly using fine-grit sandpaper or steel wool to remove buildup and ensure proper functionality.

Why wont my furnace kick in?

There are several reasons why your furnace may not turn on. The most common include a broken thermostat, gas line issues, clogged air filters, blocked ductwork, a tripped circuit breaker, and issues with the furnace itself.

Can a thermostat cause a furnace to not ignite?

Furnace Not Igniting Because of Incorrect Thermostat Setting

Make sure the thermostat is set to heating and that the temperature is set high enough to actually turn the furnace on. It may be set at too low of a temperature to activate the furnace.

Can a flame sensor work intermittently?

Short cycling is a common sign of a bad flame sensor. The sensor thinks that there isn't a flame, so it shuts off the gas as a safety mechanism.

Why won't my furnace ignite?

If your furnace won't come on, check the ignition sensor. A dirty sensor could be the reason why your furnace won't stay on or even why your furnace won't stop blowing cold air. Only if the furnace's heat exchanger — the part that actually heats the air — senses an ignition, will it power on the furnace.

Is a furnace flame sensor the same as a thermocouple?

Flame sensors and thermocouples are both furnace safety components that sense heat and prevent gas leaks. However, the two parts function differently and are not interchangeable. Thermocouples are used in furnaces with pilot lights; flame sensors are used in furnaces with electronic ignitions.

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.

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.

How do I get my furnace to kick on?

If nothing's happening when you're expecting the whir of a blower, check the circuit breaker and fuse. At your electrical panel, make sure all switches related to the HVAC system are on. Reset any tripped breakers and replace all blown fuses, and your furnace should kick back on.

How do I reset my furnace ignitor?

You can reset the ignition on your furnace simply. To do this you must cut the power to the furnace first by turning off the circuit that powers it. Once the power is off you can remove the burner door to reveal the ignitor. Turn this to “off” and leave it off for at least 5 minutes.

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