How To Bypass A Flame Sensor On A Furnace (Do This!)

Jessica Stone
by Jessica Stone

If the flame sensor on your furnace is broken or faulty, you may be looking for a temporary solution. Those who live in especially cold areas of the country rely on their furnace to keep them warm during winter months. Since your flame sensor is a safety device, when it’s not working properly, your furnace will not operate.

If this describes your current situation, you may find yourself asking if it’s possible to bypass the furnace’s flame sensor. In short, you cannot bypass the flame sensor and manually light your furnace. Bypassing safety controls, even when possible, should never be done. It is hazardous, extremely dangerous, and will certainly risk your life and safety.

Instead, you should try cleaning or replacing your flame sensor to get it and your furnace back to good working order.

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What is a Furnace Flame Sensor?

Losing heat on one of the coldest days of the year is an absolute nightmare for those who live in cold areas of the country. While we know how essential furnaces are, many homeowners are unfamiliar with its different mechanisms and how important they can be. This is especially true for the furnace’s flame sensor, a vital safety feature that ensures the gas fueling your furnace is burned properly and not leaking into your vent system.

The sensor’s job is to sense when a flame is burning inside your furnace’s burner assembly. If no flame is detected, the sensor will shut off the furnace to prevent unburned gas from escaping into the chimney or vent system. In furnaces that are older, or have draft inducers, this can cause an accumulation of unburned fuel and a “hard ignition” (a type of minor explosion) that will result in damage to the furnace.

How Does a Flame Sensor Work?

Physically, the flame sensor is a small, metallic rod with a base made of porcelain that conducts electricity. At the start of your furnace’s heating process, the gas valve opens and the electrical current detects if a flame is present or not. It does this by sensing the heat from the flame.

If it does detect heat from the flame, the gas proceeds with igniting and the heating cycle continues. However, if the flame sensor does not sense the presence of a flame within 10 seconds, it will shut down the furnace entirely.

Most often, the heating cycle repeats one or two more times, but if the flame is not detected your furnace may be “locked out” and need to be reset. Your furnace can generally be reset by shutting off the power momentarily or shutting off the furnace itself at the thermostat, waiting a few moments, and then turning it back on.

Signs Your Flame Sensor is Bad

When it comes to your HVAC system, catching an issue before it occurs is always a plus. Thankfully, diagnosing a bad or failing flame sensor is relatively simple. Here are some tell-tale signs that you have a bad flame sensor in your furnace:

  • The tip of the flame sensor has visible rust, dust, or soot.
  • The porcelain base of the flame sensor is chipped, cracked, or completely broken.
  • The furnace lights but then the gas burners go out almost instantly (short cycles).

How to Bypass a Flame Sensor on Furnace

If your flame sensor has caused your furnace to shut down, you may be wondering if you can bypass the sensor itself. The first thing you can try to get around this is by resetting the furnace. As previously mentioned, this can be done by shutting off the furnace at the thermostat and then turning it back on or shutting off the power briefly. However, if the flame sensor is broken or faulty, simply resetting the furnace will not resolve the issue.

If you’re trying to temporarily bypass your flame sensor to get through a particularly cold evening, you may be out of luck. Unfortunately, you cannot bypass the flame sensor and light your furnace manually, as the gas valve will not open until the flame igniter sends a signal to the control board. Attempting to manual light your furnace is very dangerous and should never be attempted.

Instead, if you suspect there is an issue with your flame sensor, you can try to troubleshoot the issue. In most cases, it either needs a good cleaning or to be replaced entirely.

How to Troubleshoot a Flame Sensor on Furnace

To troubleshoot the issues you are having with your flame sensor, you first need to locate the component. First, you will need to remove the furnace access cover which may be held together by either knobs, screws, or tabs and slots. Once removed, the flame sensor should be immediately visible, mounted to the outside of the burner assembly.

It appears like a metal rod with a white or dull yellow/brown porcelain material on the mounting end, with a single wire attached to it. While some are straight, other flame sensors have a 45-degree or 90-degree been on the end. The flame sensor will jut into an open fire chamber or through the housing into the interior where the flame is burning.

Once you’ve located the flame sensor, follow these steps to diagnose and troubleshoot the problem:

  • First, cut the electrical power to the furnace at the electrical box or the furnace itself.
  • Locate the gas valve and close it.
  • Remove the screw that is mounting the flame sensor and gently remove it from the assembly.
  • Visually inspect the flame sensor for damage or debris.
  • If it is intact but appears burnt, dirty, or has corrosion, it may just need a basic cleaning (see below).
  • If any part of the porcelain insulation is cracked or pieces of the component appear to be missing, the flame sensor may need to be replaced (see below). It may also need replacing if the cleaning does not remedy the issues.

Based on your inspection, proceed accordingly with cleaning or replacing the device. Then, remount the flame sensor, restore power, reopen the gas valve, and light the furnace.

How to Clean a Flame Sensor on Furnace

If you discovered corrosion, dirt, or burn marks on your flame sensor, it may need a simple cleaning in order to function properly again. To safely and effectively clean your flame sensor, follow these steps:

  • Completely shut off the power to the furnace.
  • Gain access to the flame sensor by opening the combustion door chamber on the furnace.
  • Remove the flame sensors mounting screw and detach the device from the assembly.
  • Clean the flame sensor using a light-grit sandpaper, steel wool pad, or emery cloth. Scrub until you’ve removed all traces of rust, dust, grit, and grime.
  • Clear out any dust in the electronic igniter and furnace pilot before replacing the sensor.
  • Once clean, reinstall the flame sensor and shut the combustion chamber door.
  • Restore power to the furnace and relight.

If the flame sensor is still not operating as expected after the cleaning, it most likely needs to be replaced.

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How to Replace a Flame Sensor on Furnace

Your local home improvement center or furnace parts suppliers should have the flame sensor that you need to replace. Bring the broken device along with you along with your furnace information to ensure that you purchase the correct one. You’ll need to know the model number, brand, and serial number, all of which can be found inside the furnace’s access panel.

If you are unsuccessful at a local supplier, there are many online retailers that sell flame sensors for a variety of different furnaces. A quick call to the supplier will generally help guide you in the right direction if you cannot find it by doing an online search. Keep in mind that you may not need an exact replacement part. You can purchase universal flame sensors at your local Home Depot, Lowes, Menards, or other big-box building supply store.

Once you secure your new flame sensor, follow these steps to replace it:

  • Shut off the furnace completely.
  • Remove the mounting screw, and detach the broken flame sensor from the assembly.
  • Place the new flame sensor into the opening.
  • Fasten it with the mounting screw.
  • Connect the wire to the new flame sensor.

It’s as easy as that! However, if you’ve cleaned your sensor, replaced it, and are still having trouble, you may want to call in an HVAC professional. Enlisting the help of a certified professional will always ensure the comfort and safety of you and your family.

Jessica Stone
Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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