Pilot Light Lit But Burners Won't Ignite [Fixed]

If your furnace’s pilot light is lit but the burners won’t ignite, that could be due to a few issues that need attention. In this blog post, we’ll be looking at the possible reasons the furnace burners will not ignite despite the pilot light being lit.

Possible reasons furnace burners will not ignite despite the pilot light being lit include a dirty flame sensor, a bad thermocouple, an open limit switch, a dirty pilot orifice, or a malfunctioning gas valve. Some of these issues can be resolved by cleaning the affected component while others may require replacement of the affected part.

Let’s take a look at the chart below that summarizes possible issues the burners will not ignite and how these issues could be fixed.

Possible reasons a Pilot light stays lit but burners won’t ignite

Possible cause	Fix
Thermostat issue	Set thermostat correctly
Dirty flame sensor	Clean flame sensor
Dirty pilot orifice	Clean pilot orifice
Thermocouple	Inspect thermocouple
open limit switch	Investigate if you have a bad limit switch

Thermostat issues

Before you begin troubleshooting why your furnace burners are not igniting, start by checking your thermostat settings. Ensure that your thermostat is set to heating mode and the temperature is raised above room temperature.

Incorrect thermostat settings occur more often than you might think, so it’s likely that the thermostat was incorrectly set.

Dirty flame sensor

A dirty flame sensor is the most common possible reason you have a furnace pilot light lit but the burners won’t ignite.

A flame sensor, or sensor rod as I like to call it, is a small straight or L-shaped piece of metal that senses if there is a flame from the burners. It is one of the furnace’s safety devices that prevents gas from leaking out in case of an ignition failure.

Over time and with age, soot and other residues from the combustion of gases from the burners can accumulate on the flame sensor, making it difficult for the flame sensor to detect flames.

If this happens, you may have the pilot light lit but the burners will not ignite because the sooty flame sensor will be unable to detect the flame and, consequently, will not open up the gas valve.

Luckily, there is an easy solution to this problem, which is simply cleaning the flame sensor. Refer to the video below. Locate the flame sensor and detach it. Use something like a currency bill to rub against it to remove the dirt. Then, place it back and turn on the furnace.

If the problem doesn’t go away, you might have to replace the flame sensor.

If the pilot light is lit but the burners won’t ignite, it’s also possible that there is no gas pressure coming to the burners, which is the next issue I am going to discuss.

Dirty Pilot orifice

Another reason your furnace burners won’t ignite despite the pilot light being lit is that the pilot orifice is too dirty.

When the pilot orifice is dirty, it will not keep the pilot generator hot enough to produce the required voltage to energize the gas. This can prevent the internal gas valve from opening to allow gas onto the burner trays.

So, how can you tell if your pilot orifice is dirty?

You can determine this by closely inspecting the pilot burner flame. If the pilot orifice is dirty, watch out for signs such as:

A soft blue pilot burner flame
Being easily blown around
Having yellow tips

If you observe any of these signs, the pilot orifice is dirty and requires cleaning. Typically, a flame from a clean pilot orifice will be a sharp blue flame, resembling a small blowtorch, with one lobe of the flame engulfing the pilot generator.

To clean the pilot orifice, you will need to disassemble the pilot burner, usually with the brass nut at the bottom of it. At this point, the pilot orifice will typically fall into your hand for cleaning.

You can clean the pilot orifice by using compressed air or a thin wire to carefully remove any dirt or debris.

Bad Thermocouple:

Another possible reason your furnace burners won’t ignite despite the pilot light being lit is that the thermocouple is malfunctioning.

A thermocouple is made of two separate metals that are joined together on one end that is immersed in the pilot flame and senses heat from it before opening the gas valve. (But this will depend on the model)

So when the thermocouple senses heat from the flame sensor it will generate a small voltage that will open the gas valve.

If the thermocouple is not properly placed or aligned, it will not touch the pilot flame. So, ensure that the thermocouple is getting immersed in the pilot flame and if it’s not try to align it properly.

Sometimes it’s the thermopile itself that will be bad. So try checking to see if the thermocouple is generating enough voltage to keep the safety valve open. If the thermopile is bad, you have to replace it.

Open High Limit Switch

The last issue, but certainly not the least, is an open high limit switch. A high limit switch is yet another safety feature that shuts off the furnace when there is overheating.

Overheating of the furnace can be caused by various issues, such as blocked vents, dirty air filters, or a defective or malfunctioning blower motor.

You can examine the high limit switch and check it for continuity. If it has no continuity, then it’s most likely defective, and you might need to replace it.

Otherwise, take the time to investigate why your furnace could be overheating.

If you cannot determine the cause, I strongly recommend that you call a local HVAC technician to come and inspect your system.