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A furnace not turning on on the first cold night of the year is one of the most common heating calls there is, and a lot of the time the fix is something you can handle yourself in a few minutes. A thermostat set wrong, a switch flipped off, a clogged filter, or a dirty sensor can all keep a perfectly good furnace from starting.
This guide walks through the 8 things to check first, in order from easiest to hardest, including the thermocouple fix that has gotten our own furnace running again more times than I can count. It also covers the safety rules to follow before you touch anything and the signs that it’s time to call a pro.
What’s in This Guide
Safety First: Read This Before You Start
A gas furnace is safe to troubleshoot as long as you follow a few rules:
- If you smell gas (a rotten-egg smell), stop. Don’t flip any switches or light anything. Get everyone out of the house and call your gas company or 911 from outside.
- If your carbon monoxide alarm is sounding, or anyone has a headache, dizziness, or nausea, get everyone into fresh air and call 911.
- Before you open the furnace or touch any part inside it, turn off the power switch and close the gas valve to the furnace.
- Never bypass a safety switch to get the furnace running.
Carbon monoxide has no smell, so a working CO alarm on every level of your home is the only way you’ll know it’s there. A combination smoke and carbon monoxide detector covers both in one device. Source: U.S. Consumer Product Safety Commission — Carbon Monoxide Information Center
What’s Happened at Our House
Almost every time our furnace hasn’t come on, the culprit has been the thermocouple. It’s a small metal probe that sits in the pilot flame, and when it gets coated in soot, the furnace thinks the pilot is out and won’t send gas to the burners. My temporary fix is to take it out and clean the tip with very fine grit sandpaper. That gets the furnace running again until I can pick up a replacement. The other time or two, the pilot light had simply gone out, and relighting it fixed the problem.
Those two problems only apply to furnaces with a standing pilot light, which is the small flame that stays lit all the time. Most newer furnaces don’t have one. They use an electronic igniter and a part called a flame sensor instead, and a dirty flame sensor is one of the most common reasons a newer furnace won’t turn on. The good news is that the fix is almost the same. Both are covered in Step 7.
8 Things to Check When Your Furnace Won’t Turn On
Work through these in order. The first four take a minute or two each and don’t require opening the furnace.
Step 1: Check the thermostat
Make sure the thermostat is set to Heat, not Cool or Off, and that the set temperature is higher than the current room temperature. Bump it up 5 degrees so you’ll know right away if the furnace responds. If the screen is blank, replace the batteries. A dead thermostat battery is one of the most common reasons a furnace won’t turn on, and it’s an easy one to overlook. Source: Bryant — Furnace Not Turning On? Causes and Fixes
Step 2: Check the power switch and breaker
Your furnace has a power switch on or near the unit that looks like a regular light switch. It’s easy to flip off by accident, especially if it’s next to a light switch on a basement stairway. Make sure it’s on. Then check your electrical panel for a tripped breaker labeled furnace. If it’s tripped, flip it all the way off and then back on. If it trips again, leave it off and call a pro. Source: Bob Vila — How to Fix a Furnace That’s Not Turning On
Step 3: Make sure the furnace door is closed tight
Most furnaces have a safety switch behind the front panel that keeps the furnace from running when the panel is off. If someone changed the filter and didn’t seat the door all the way, the furnace won’t start. Push the panel firmly into place until it sits flush.
Step 4: Check the air filter
A clogged filter restricts airflow, which can make the furnace overheat and shut itself off as a safety precaution. Pull the filter and hold it up to a light. If you can’t see light through it, replace it with a new furnace air filter in the size printed on the edge of your old one. Most filters should be changed every 1 to 3 months during heating season. Our guide on how to choose a furnace filter explains what MERV ratings mean and which one to buy. Source: Bryant — Furnace Not Turning On? Causes and Fixes
Step 5: Make sure the gas valve is open
Find the gas line running into the furnace. There’s a shutoff valve on it, usually within a few feet of the unit. The valve is open when the handle runs parallel to the pipe and closed when it sits crosswise. If it’s closed, open it. If you have other gas appliances, like a stove or water heater, check that they’re working too. If none of them are, the problem may be with your gas supply, so call your gas company. Source: Bob Vila — How to Fix a Furnace That’s Not Turning On
Step 6: Check the pilot light (older furnaces)
If your furnace has a standing pilot, look through the small viewing window near the bottom of the furnace for a small blue flame. If it’s out, relight it by following the instructions on the label inside the furnace door. Every model is a little different, but most have you turn the gas control knob to Pilot, hold it down, and light the pilot with the built-in igniter button or a long-reach lighter, then keep holding the knob for 30 to 60 seconds before releasing it and turning it to On.
This fixed our furnace once or twice. But if the pilot lights and then goes out as soon as you let go of the knob, the thermocouple is almost always the reason. That’s Step 7.
Step 7: Clean the thermocouple or flame sensor
This is the fix that has worked for us most often. Both parts do the same job: they tell the furnace that a flame is actually burning, so it’s safe to send gas to the burners. When they get coated with soot or residue, they can’t read the flame, and the furnace shuts the gas off as a precaution. Source: Bryant — Furnace Not Turning On? Causes and Fixes
On a furnace with a standing pilot, the part is a thermocouple. It’s a thin copper tube with a metal tip that sits right in the pilot flame. The heat from the flame makes a tiny electrical signal that holds the gas valve open. A dirty tip weakens that signal, and the pilot won’t stay lit. Source: eFireplaceStore — How to Clean a Thermocouple
On a newer furnace, the part is a flame sensor. It’s a thin metal rod, usually bent, that sits in front of the burners and is held in place by a single screw.
To clean either one:
- Turn off the power switch and close the gas valve to the furnace, then let it cool for at least 20 minutes
- Remove the front panel and find the thermocouple (in the pilot flame) or flame sensor (in front of the burners)
- Remove the part. A flame sensor comes out with one screw. A thermocouple unscrews from the gas valve and slips out of its bracket
- Gently rub the metal tip with very fine sandpaper or emery cloth until it’s clean and lightly shiny. Go easy, especially on a flame sensor, so you don’t bend or snap the rod from its base
- Wipe off the dust, reinstall the part, put the panel back on, open the gas valve, turn the power on, and relight the pilot if you have one
Cleaning is a temporary fix, and that’s exactly how I treat it. If your furnace keeps going out, or the pilot still won’t stay lit after cleaning, the thermocouple is worn out and needs to be replaced. A universal thermocouple is an inexpensive part that fits most furnaces with a standing pilot. Match the length to your old one. If you’re not comfortable working on the gas valve, have a pro handle the swap.
Step 8: Read the blinking status light
Most furnaces built in the last 20 years or so have a small status light on the control board, visible through a little window in the lower panel. If it’s blinking, it’s flashing an error code. Count the blinks and match the pattern to the chart printed inside the furnace door. It will tell you whether the problem is the flame sensor, the igniter, a pressure switch, or something else.
On high-efficiency furnaces, one common code is a clogged condensate drain. These furnaces make water as they run, and if the drain line or trap clogs, a safety switch shuts the furnace off. Write down any code before you call for service. It saves the technician time and saves you money. Source: Bryant — Furnace Not Turning On? Causes and Fixes
When to Call a Pro
Stop troubleshooting and call a licensed HVAC technician if:
- You smell gas at any point
- Your carbon monoxide alarm goes off
- The breaker trips again after you reset it
- The pilot won’t stay lit even after you clean or replace the thermocouple
- The status light shows a code for the control board, gas valve, igniter, or heat exchanger
- You hear loud banging, screeching, or grinding when the furnace tries to start
- You’d need to work on the gas line or gas valve and aren’t comfortable doing it
If it’s below freezing and you’re waiting on a repair, keep the cabinet doors under your sinks open and let a faucet run at a thin stream. Our guide on how to prevent frozen pipes covers the rest.
How to Keep It From Happening Again
Most furnace no-starts come down to a few simple things you can stay ahead of:
- Change the air filter every 1 to 3 months during heating season
- Replace the thermostat batteries every fall, before you turn the heat on for the first time
- Turn the heat on for a test run on a cool fall day, before you actually need it
- Keep the area around the furnace clear of boxes and storage so it gets plenty of air
- If you have a standing pilot, keep a spare thermocouple on hand. It’s a cheap part, and having one ready turns a cold night into a 20-minute fix
- Have the furnace inspected by a pro once a year
A smart thermostat can also send an alert to your phone if your home’s temperature drops, so you’ll know the furnace has stopped even if you’re away.
Frequently Asked Questions About a Furnace Not Turning On
Why is my furnace not turning on but the thermostat is working?
If the thermostat has power and is set to Heat, the problem is usually at the furnace. Check the power switch, the breaker, the furnace door, the filter, and the gas valve, in that order. If those are all fine, a dirty thermocouple or flame sensor is the next most likely cause.
How do I reset my furnace?
Turn off the power switch on or near the furnace, wait about 30 seconds, and turn it back on. Some furnaces also have a red reset button on the blower motor. Press it once. If the furnace shuts off again, don’t keep pressing it. A repeated shutdown means something is wrong, and resetting it over and over can make it worse.
How do I know if my thermocouple is bad?
The classic sign is a pilot light that lights but goes out as soon as you let go of the gas control knob. If cleaning the tip with fine sandpaper doesn’t fix it, or it only works for a short time, the thermocouple needs to be replaced.
Can I clean a flame sensor with sandpaper?
Yes. Use very fine sandpaper, emery cloth, or fine steel wool, and rub the metal rod gently until it’s clean. Turn off the power and gas first, and be careful not to bend the rod or break it off its ceramic base.
Why does my furnace turn on and then shut off?
A furnace that lights and then shuts off within a few seconds usually has a dirty flame sensor. The burners light, but the sensor can’t confirm the flame, so the furnace shuts the gas off for safety. Cleaning the sensor fixes it most of the time. A clogged filter can also cause the furnace to overheat and shut off after running a few minutes.
How much does it cost to fix a furnace that won’t turn on?
If the fix is a thermostat battery, a filter, or cleaning a sensor, it costs little or nothing. A thermocouple or flame sensor is an inexpensive part. A service call costs more, especially on nights and weekends, which is why it’s worth working through this checklist first. Prices vary by region and by what the technician finds.