Furnace pilot light won’t stay lit – Causes & Cures

This furnace troubleshooting guide will explain why your furnace pilot light won’t stay lit, how to resolve it and whether it might be time for furnace replacement. We’ll say the obvious. If your furnace has a pilot light, it is old and probably doesn’t have too many winters left.

Furnace Pilot Light Keeps Going Out – Causes & Cures

Let’s look at the most common reasons the furnace pilot won’t stay lit. That will be followed by an explanation of how to light a pilot light on a furnace.

Cause #1: Dirt

Does the pilot light fire at all? If so, what color is the flame?

A pilot light should be a rich blue color, a nice blue cone. That’s the sign of a clean pilot light orifice.

Is it yellow or yellowish-orange? If it is blue, is it small and flickering? Those are signs of dirt in the orifice impeding the gas, making the flame weak and potentially preventing it from staying lit.

The Cure: The solution is to clean out the orifice or replace it. Shut off the gas valve, and use a fine needle clean the small hole in the orifice through which gas passes. You can also remove the tube from the gas assembly, and blow through it. An air compressor or compressed air in a can might work too.

This video shows how to clean a pilot light on a gas water heater, but the process is the same with a gas furnace.

If you’d rather start fresh, remove the pilot assembly and take it to a parts store or find a replacement online. Orifices start at less than $10; pilot assemblies are $25-$50.

Pro Tip: If you aren’t experienced working with gas appliances, contact a local furnace company. The cost to clean a furnace, including the pilot orifice, starts below $100.

A clean furnace runs more efficiently, so you’ll spend less each month on your gas bill. Furnace maintenance is also a good way to reduce the likelihood of expensive future repair issues.

Cause #2: Bad Thermocouple

A thermocouple is a nifty invention that senses a flame, in this case, the pilot light.

Why? If there’s no flame and the furnace gas valve remains open, you’ve got a gas leak and the potential for a deadly explosion.

If the thermocouple is cracked or worn out, dirty or improperly lined up, it won’t signal to the gas valve that there’s a flame, and the gas valve will close. No gas, no pilot light.

Here’s what a thermocouple looks like.

The Cure: First, check to see if the thermocouple flame sensor is directly in the pilot flame or where the flame should be. A misaligned flame sensor is the rarest problem, but easiest to spot and solve. If the furnace or thermocouple got jarred, the thermocouple might be knocked out of position. Gently move it back into position, and make sure the screw holding it is snug.

If it’s lined up and the pilot won’t stay lit, clean the thermocouple. A very dirty thermocouple won’t sense heat. Remove the heat sensing end, and clean it with emery cloth.

This video gives a great explanation of what the thermocouple does and how to clean it.

If cleaning doesn’t work, then you can test the thermocouple using a multimeter. Set the meter to DC and millivolts. Attach one lead to the end furthest from the flame sensing part of the thermocouple. Attach the second lead to the copper tube a few inches up the line.

Then use a BBQ lighter to apply heat to the flame sensor. As it heats up, it generates millivolts – typically between 18 and 35. If your meter isn’t moving or won’t get to 18, the thermocouple is probably bad.

If you’re handy and take safety precautions, it can be a DIY job. The part costs less than $25, and universal thermocouples are available.

This 36” Honeywell universal thermocouple is long enough to fit most furnaces. If your thermocouple is a different length, select the link and then type into the Search Box “universal thermocouple” followed by the length needed. Common lengths are 18”, 24”, 36” and 48”.

The first two causes, a dirty gas orifice and an issue with the thermocouple account for 90% of pilot flame issues. There are a couple of other things it might be.

Cause #3: A Draft

The clue to this cause is that the pilot light lights easily after being out and stays lit for a few hours to a day or more. But if the furnace pilot won’t stay lit and you can determine a cause within the furnace, a draft might be the cause.

Is the pilot light getting blown out by a draft through a window left open or a door ajar?

Has the wind been strong – and blowing toward the furnace vent on the outside of the house? That’s sometimes enough to cause air to travel down the vent pipe and blow out a pilot light.

If the problem is intermittent, keep an eye on wind conditions to see if there is a connection.

Cause #4: A Bad Gas Regulator

The gas regulator is on the outside of your home. It is attached to the gas line and regulates the flow of natural gas or propane to be proper for the furnace and other appliances.

If the regulator is damaged, it might not allow enough gas to flow through it. A bent/crimped gas line, more common with propane, is also possible.

Do you have other gas appliances? If so, are you having issues with them? The gas dryer isn’t hot? The flame on the gas range is weak? Does the furnace pilot light go out while you’re using another gas appliance? The pilot isn’t getting enough gas.

If any of these fit, the gas regulator or gas line is probably the problem.

The Cure: This is obviously a case for your gas provider. Call them, and they’ll send someone out to check the regulator and line and make needed repairs.

How to Light a Pilot on a Furnace

Is the gas valve turned on? Check.

Is the power to the furnace on? Make sure the switch on or near the furnace is in the “on” position and that the circuit in the electrical panel is “on.” You can also check this by turning the thermostat to the “Fan” mode if it has one, and the furnace fan should come on.

OK, the pilot is out and you want to know how to light a pilot light on a furnace to see if you can get some heat in your cold house.

Here are step by step directions for lighting a furnace pilot light.

The first thing to do is to check the inside panel or the side of your furnace, or the owner’s manual, to see if the manufacturer has provided its own step by step pilot lighting instructions.

Follow the manufacturer’s instructions. If there are none, these “universal” instructions should work.

Step 1: Turn the thermostat off. This will prevent the thermostat from calling for heat and the gas valve opening while you’re lighting the pilot – and it might allow you to keep your eyebrows.

Step 2: Locate the gas valve knob printed with On, Off and Pilot. This valve is part of the larger pilot light assembly, which can be seen in the video above about a Bad Thermocouple. The gas line goes through the assembly, and the gas tube to the pilot light connects to it as well.

Step 3: Turn the knob to the Off position. Wait a few minutes before taking the next step. This will allow any raw gas to dissipate before you light anything. Of course, do not light anything if you smell gas.

OK, the next 4 steps are where you get it done.

To prepare for them, get a BBQ lighter or a BBQ skewer and tape a match to the end of it.

Step 4: Turn the knob to Pilot.

Step 5: Light the BBQ lighter or the match on the skewer, and hold the flame within about an inch of the pilot light orifice.

Step 6: Depress the gas button to allow gas to flow to the pilot orifice. The button might be labeled “Reset” or be red or both. Hold the button down until the pilot lights.

Step 7: While still holding down the button, move the knob from Pilot to On. Then let up on the button. If the furnace pilot light won’t stay lit, try it again, and keep holding the button down for a few seconds before moving it from Pilot to On.

Here’s the clearest tutorial video on how to light a furnace pilot light. It explains these steps, and the video gives great visual instruction.

If it stays lit, great. If the furnace pilot keeps going out, then:

1. Follow the troubleshooting guide above.
2. If you’ve done that or don’t want to DIY, then contact a furnace repair company.

Feel free to use the Free Local Quotes form or phone number on this page to get competitive quotes from licensed, insured and experienced technicians in your area.

If you want to explore more options on your own first, see our Furnace Troubleshooting Guide to determining common furnace problems and their solutions.

Is It Time to Replace Your Furnace?

Furnace manufacturers began replacing pilot lights with electronic ignitions in 1990, though furnaces with pilots were made for a few years after that. In short, if your furnace has a pilot, it has already seen a lot of winters and might be prone to quit on you again and for good during a cold winter stretch.

Replace and Save: We recommend considering pre-emptive furnace replacement as a way to save money, especially if you plan to live in the home long-term.

Think about it this way. Furnace repair costs can mount up. And a furnace as old as yours is going to require a replacement in the next 3-5 years almost for sure. Instead of spending money repairing the furnace and then spending even more in a few years to replace it, the savvy financial decision could be to replace it now.

The truth is, some unethical furnace repair companies are happy to keep repairing an old furnace, milking it for all the repair dollars they can and earning your trust along the way. Then, next winter, or maybe the year after, they give you the bad news that the furnace has to go, and you need a replacement.

Again, if the furnace is old, which it is, and especially if you’ve already put money into repairs, consider replacing it.

There are advantages to furnace replacement vs furnace repair:

1. The furnace will be new and unlikely to break down for many years.
2. It will be under warranty, usually a 10-year parts warranty and lifetime or 20-year heat exchanger warranty.
3. It will probably be more efficient – possibly 15% to 25% more efficient – and you’ll see an instant decrease in your gas bill.
4. Newer furnaces have performance features that improve indoor comfort – keeping temperatures balanced, and if you have central AC, removing more humidity in summer.

Hopefully this pilot light troubleshooting information has helped get your furnace going and been useful as you consider next steps. Feel free to leave a comment or question or to share this post using the social media options. Thank you for reading!