Troubleshooting A Gas Heater That Won't Stay Lit - HouseholdAir (2023)

It’s not too uncommon. Perhaps the pilot light cannot stay lit. Maybe it goes off within seconds of coming on, not even allowing enough time to light the burner. Or maybe it’s the burner that keeps going every few minutes.

Although it can be irritating, you can do something about it. However, you first need to pinpoint the problem. Below we look at common reasons why your heater may struggle to stay lit and possible solutions.

How the Gas Furnace Ignition System Works

Before we begin looking at the potential causes, it’s helpful to understand the basics of a gas furnace ignition mechanism. Although gas furnaces come in a variety of styles, shapes, and designs, all gas furnace ignition systems operate as follows;

  1. The thermostat calls for heat.
  2. The inducer fan kicks ON. This fan is responsible for bringing in cool air from your rooms into the furnace for heating. It helps create a safe combustion chamber.
  3. The gas valve opens, allowing propane or natural gas to flow to the pilot burner.
  4. Upon detecting the presence of gas, the pilot ignitor lights via an electric spark or hot surface ignition.
  5. Upon sensing the flame on the pilot, a flame sensor signals your furnace that there’s a flame to burn.
  6. The gas valve sends even more gas to the furnace burners.
  7. The pilot light lights the main burners, leading to full room heating.

Essentially, that’s how a gas furnace ignites, though there might be a few differences depending on the design. For instance, direct ignition furnaces don’t include steps 3-5. Instead, you can light the burner directly.

Furnaces with standing pilot lights are also slightly different. They skip step #4 because the pilot light remains lit throughout.

Now that we understand how furnace ignition systems work, it will be easier to understand what could go wrong with your system.

Why Won’t Gas Heater Pilot Light Stay Lit?

Let’s begin with the pilot light. What’s to blame when the pilot keeps going out every few seconds, and what can you do about it? Perhaps you only see the pilot flame for a few seconds before it quickly goes out. Or maybe it stays on a little longer but not long enough to light the furnace burners.

The exact problem will likely depend on the type of ignition system. However, generally, here’s what could be wrong;

1. The flame sensor is bad

As we’ve seen, the flame sensor tells your furnace that the furnace has a flame, thus triggering the release of gas to the burners. However, if the flame sensor malfunctions, it may fail to signal the gas valve to release gas even if a flame is detected.

Flame sensors can lose flame sensing capacity for several reasons. For instance, a flame sensor may be become worn naturally. It may also become damaged.

Diagnosing a malfunctioned flame sensor requires a multimeter to detect current flow through it. In most cases, you’ll need to replace the damaged/compromised thermocouple.

2. Poor flame sensor positioning

Another reason the pilot flame may go out without lighting the main burner is the incorrect flame sensor positioning. The flame sensor must be close enough to the pilot flame to detect the flame.

If it’s too far away, it may fail to detect the presence of the flame. Or, it may only detect a weak flame. In both cases, it will likely send a negative response to the gas valve.

The main reason the flame sensor might become wrongly positioned is dirt buildup. Too much debris can push the thermocouple rod away from the flame. Tampering is another possible cause.

Fortunately, you can fix flame sensor positioning without involving an HVAC professional. However, don’t hesitate to seek help if you’re not confident in your DIY skills.

3. The gas valve has issues

As we’ve seen, once the flame sensor signals the presence of a flame, the gas valve springs into action to provide sufficient gas for the heating process. However, imagine in an instance where the gas valve is compromised!

Imagine a situation where it’s blocked or shut off. It wouldn’t be able to send enough gas to burners. Thus, the flame sensor will likely instruct the furnace to shut off the pilot flame.

Gas valves can become clogged due to dirt buildup. They can also become bent if hit by a blunt object. Or it may be in good shape but turned off.

Begin by inspecting the valve for dirt and debris buildup and wipe off any dirt as appropriate. Also, make sure it’s ON. Call the pros if you notice dents to the gas line or hear hissing gas (a sign of a leak).

4. The pressure switch has flipped

The pressure switch is a safety feature designed to protect you and your family from gas poisoning. It closely monitors gas venting in your home and shuts down the gas supply if it detects that the venting in your heating system isn’t operating correctly.

Poor gas furnace venting may result from several issues. For instance, blocked air filters can result in poor air circulation in the home. Blocked air ducts can also impede venting.

It’s advisable to fix the issue causing the pressure switch tripping before you switch it back ON. Otherwise, it will just trick OFF again.

5. It’s an electrical issue

If you’ve tested all the above, but the problem persists, it might be time to check the electrical controls. One particular place to check is the ignition board. The ignition board coordinates the entire ignition system, telling each part what to do and when. It relays information from one part of the ignition system to the other and from/to the rest of the ignition system.

If the wires connecting the different parts of the circuit relay are damaged, the circuit board will likely fail. The same applies if you have a blown fuse.

The bad news is that electrical issues are a little harder to fix as you risk worsening the damage. So, it’s best to call an electrician or HVAC pro.

Burner Ignites but Goes Out Without Heating the House, Perhaps Repeatedly

At this point, you’ve ruled out pilot light issues as the pilot stays ON long enough to light the burner. However, the burner won’t stay lit. Maybe it stays on for five minutes before going out. Or perhaps it goes out right after coming on and does so repeatedly.

The following are a few likely causes and probable solutions.

Clogged/blocked burners

The first thing you want to check is whether the burners get gas in the first place. If they don’t, then the most likely issue is a clogged or blocked burner. A common sign of blocked burners is an unhealthy flame that’s yellow with lots of smoke. Clogged burners may also have an irregular, weak flame.

Clogging is a natural process that occurs over time. Dirt and debris will collect around the furnace, including around the burner. Over time, this can block the tiny holes through which gas comes out of the burner.

The simplest remedy to clogged burners is regular maintenance. Make sure to clean your burner regularly to brush away and dirt.

Thermostat issues

If the burners seem okay, take a step back and determine if the thermostat is working and set correctly. Thermostats are the brain of HVAC systems. They tell the heater when to heat and when to stop. They also determine the right level of heating depending on indoor air conditions. Thus, even a small thermostat mishap can cause significant confusion.

The two main things to check about your thermostat are functionality and setting. Is it working? Is it ON? And, is it set correctly? If it’s wrongly set, it may mislead the heater and cause the unit to stay OFF even when attempting to ignite it.

If the thermostat is dead, replace it. If it’s damaged, see if an HVAC pro can fix it. However, if it’s merely a wrong setting, you can fix that with ease.

Limit switch issues

Finally, you may also want to check your limit switches. All gas furnaces are designed with safety mechanisms to protect the unit and you and your family.

These safety controls particularly monitor gas leaks, pressure fluctuations, and overheating. Some also monitor the unit for flame rollout. Typically, they will cut the gas supply if an issue is detected.

Take an example of the flame rollout switch. It closely monitors the furnace for flame rollout and overheating and instantly disengages to cut off the gas supply if an issue is detected.

More importantly, it usually remains in the OFF position until you manually fix the issue, meaning that the furnace will not light no matter how many times the pilot ignites.

The best solution here is to fix the underlying issue. If it’s an overheating problem, fix the root causes before switching back the limit switch. The same applies to gas leaks and flame rollout.

Gas furnaces are highly delicate appliances that require cautions handling. Hopefully, you’ve learned a few things about fixing common furnace issues.


Why is my gas heater not staying alight? ›

If your pilot light won't stay alight, it might be the thermocouple. Sometimes dust buildup collects on the thermocouple and blocks the sensor. Try cleaning the thermocouple and then relighting the pilot light. If it continues to go out, the thermocouple might need to be replaced.

Why does my heater ignite then shut off? ›

The typical heating cycle lasts 10-15 minutes. If your furnace shuts off after a few minutes and starts more than eight times an hour, it may require repair. Common causes of furnace short cycling include restricted airflow, a malfunctioning flame sensor, or a poorly placed thermostat.

How do I know if my thermocouple is bad? ›

If the pilot flame ignites but goes out after you release the gas control knob, the cause may be a dirty or defective thermocouple. If the gas is on but the flame will not ignite at all, a pilot tube obstruction is the most likely issue. Remove the pilot tube from the gas valve and spray compressed air to clear it.

Why does my heater not stay lit? ›

Common reasons include a nearby draft, dirt buildup, or a malfunctioning thermocouple. Another pilot light drawback is energy waste. Since the pilot light needs to remain active, your furnace is always consuming some gas. It's not a lot—usually just a few therms of gas a month–but that obviously adds up over time.

How do you clean a thermocouple on a gas heater? ›

Yes, you can clean a thermocouple, but the task must be done with a light touch. Remove the thermocouple from the process assembly, using a steel wool or emery cloth, lightly clean the tip removing any build up. Cleaning a thermocouple is not a fix for an issue, such as inaccurate readings.

Why does pilot flame keep going out on gas heater? ›

Faulty Thermocouple – If the pilot light goes out, it's up to the thermocouple to cut the gas off. If it's dirty or bent, it may be shutting off the gas unnecessarily.

Why does my furnace burner turn off after a few seconds? ›


If the burners visibly ignite, but then go out after approximately 6 seconds, this may indicate a dirty flame sensor. Multiple failed ignition attempts will typically generate an "Ignition Failure" diagnostic on the ignition control board.

Why does my heater only turn on and off quickly? ›

Pay Attention to Your Air Filters

One of the most common causes of short cycling is also the easiest to fix. If you don't make a habit to regularly clean or change your heating system's air filter, it could have a major impact on its efficiency. Warm air can't pass through the filter and it backs up within the unit.

Will pilot stay lit if thermocouple is bad? ›

A thermocouple is a function on your water heater that senses the heat on the pilot light and allows gas to flow to the burner. A bad thermocouple will shut off gas to both the pilot and the burner, which is when people notice that the water heater pilot light is out.

How much does it cost to replace a thermocouple? ›

If your thermocouple goes bad, there is only one thing to do: Replace it. If you hire a professional plumber to do this, it may cost up to $200 total. This is another repair you may wish to attempt on your own. The part itself should only cost $20 or $30.

Can I replace a thermocouple myself? ›

It's easy to replace and a new one costs $5 to $10. Rather than trying to unfasten the thermocouple inside the water heater's burning chamber, it's easier to remove the whole burner and thermocouple assembly.

Why is my pilot light not staying on? ›

Dirty or Damaged Thermocouple Explains Why Pilot Light Keep Going Out. A faulty, misaligned, dirty, or damaged thermocouple is often the culprit behind your pilot light going out. The thermocouple is an automatic safety shutoff device that prevents the gas valve from opening if the pilot light is out.

Why do I have to keep flipping the switch on my furnace? ›

Overheating. If the air inside your furnace gets too hot, the limit switch trips to prevent overheating. If the furnace high limit switch trips due to overheating, there is likely an airflow issue that is preventing proper air movement through the system.

Why does my pilot light go out when I release the pilot light knob? ›

If you try to relight a pilot light and the thermocouple is bent in a way that causes it to not make contact with the pilot light, the flame will instantly go out when you release the pilot control knob since the probe won't detect the heat from the flame.

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