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

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.

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.

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.

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.

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:

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: