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Furnace Not Working but Fan Is? Here’s What To Do

Your house is getting colder, but the air is still blowing through the vents. There are a few common problems that cause a furnace to malfunction in this way. Fortunately, there are easy ways for you to check what the problem is and implement a solution. 

If your furnace isn’t working, but the fan is, there’s a problem with the air filter, the high limit switch, or the ignitor. You can check which of these is at fault and fix it or replace the part. These errors are usually the result of equipment age and usage or improper furnace maintenance. 

This article will explain the most common causes of this problem in their order of likelihood. During these explanations, I will explain how to diagnose these problems and discuss the functions of these different parts of the furnace and some basic maintenance to prevent future problems from occurring. 

Problem #1: The Air Filter

The most common problem that causes your furnace to not work despite the fan working is an incorrectly installed or clogged up air filter. In both of these scenarios, the airflow through the filter will be insufficient, so the furnace will not turn on. Insufficient air flow would cause the furnace to overheat, which in turn can cause cracks in the furnace or even a fire, so this is a preventative function to keep you safe. 

Is the Air Filter Installed Correctly?

Where Is the Air Filter?

Your furnace filter is a thin rectangular box about 20x20x1 inches(51x51x2.5 centimeters), though dimensions vary. The dimensions of your filter will be printed on its side. It is located in the filter cabinet or tray between the furnace and the return air duct, which pulls the air from your house to circulate through the system. 

How Should It Be Installed?

There are arrows on the filter which indicate the direction of airflow, so your filter should be installed so that the arrows are pointing towards your furnace, away from the return air duct. If your filter isn’t installed this way, simply turn it around so that it’s facing the correct way and restart your furnace. 

Is the Air Filter Clean? If Not, Replace It

How Can I Tell if the Filter Is Clean?

A clean air filter is white and free of build-up. You’ll know it’s dirty if it’s gray and covered in the build-up, which means it needs to be replaced. Some common factors that cause the air filter to become dirty faster include pets, smokers, or frequent candle burning in the household. 

How Often Should the Filter Be Replaced?

The typical household should replace their furnace air filter once every three months, but you should do so more frequently if you have any aforementioned factors. I recommend Filtrete Air Filters – make sure to double-check the dimensions of your filter before ordering a replacement. I also recommend using the subscribe & save purchase option to make sure you always change your filter on schedule. 

Filtrete 20x20x1, AC Furnace Air Filter, MPR 300, Clean Living Basic Dust, 6-Pack (exact dimensions 19.69 x 19.69 x 0.81)

Problem #2: The High Limit Switch

The second-most common problem that causes your furnace to not work despite the fan blowing is a malfunction with your high limit switch. The limit switch measures the temperature of the airflow moving through the furnace and turns it on or off accordingly. 

If there is not enough airflow through the system, as caused by a dirty or improperly installed air filter, the limit switch will keep the furnace switched off in order to prevent overheating. If you’ve checked the filter and it isn’t the cause of the problem, then there may be an issue with the limit switch itself. 

Where Is the High Limit Switch?

The limit switch is located in the furnace unit so that the sensor is behind the ductwork, where the hot air is. The front of it is recognizable by the two wires coming out, which create the circuit to the sensor. 

How Do I Check if the High Limit Switch Is Working?

You can check if your limit switch is working using a multimeter. With this, you can check for voltage or continuity (resistance). I recommend the AstroAI Digital Multimeter

AstroAI Digital Multimeter with Ohm Volt Amp and Diode Voltage Tester Meter Continuity Test (Dual Fused for Anti-Burn)

Check for Continuity

Before you check continuity, make sure the furnace is off. Set your meter to continuity and unplug both wires leading into the switch. Put each meter lead on each high limit switch terminal simultaneously, and it will read out the resistance if the circuit is closed like it is supposed to be, or it will read OL (open circuit), meaning the limit switch is stuck open. 

Check for Voltage

To check voltage, make sure the furnace is on. The motor will turn on, and then you can put the meter leads to the terminals simultaneously, and it should read zero – meaning that the circuit is closed and no voltage is being lost through the switch. If you put on one lead or the other at a time, grounding the other one against another screw in the furnace, you will read out a voltage difference because you have opened the circuit. 

How To Fix a High Limit Switch

If your limit switch is malfunctioning because the circuit is open, sometimes just hitting the face of it a few times with something sturdy will be enough to hit the metal plates inside the sensor back together, which will close the circuit. Reset your furnace and see if it works. If not, you will have to remove the limit switch and replace it.

Problem #3: The Ignitor

If your filter and limit switch is working and your furnace still won’t work, the last problem you need to check for is whether your ignitor is working properly. 

Where Is the Ignitor?

The ignitor is a short metal tube grounded in ceramic with an electrical wire coming out of one end with a Hot Surface Ignitor, or glow stick, on the other end. This glow stick can go above 1800°F (982°C) and is what lights the flames. You will find the ignitor on one end of your row of burners. 

How Do I Know if It’s Working?

To check if your ignitor is working, open the furnace and turn it on and watch how it starts. The inducer motor should startup first, which will close the pressure switches, and then the ignitor should startup. If the ignitor is working, you will see it start to glow. 

If it remains dull and the burners aren’t lighting, then you need a new ignitor. Without a working ignitor, the gas valve will still open for a few seconds until the flame sensor kicks in and shuts the gas valve. If you replace the ignitor, this will fix your furnace.

How To Manually Light the Furnace

If you’re facing a couple of cold days ahead while you wait for your new ignitor to arrive, you can manually light the furnace, but this should only be attempted by experienced individuals. This video by Word of Advice TV explains how to manually light a furnace: 

Final Thoughts

The three most common problems causing a furnace to not function while the fan continues to blow are: 

  • A backward or dirty air filter
  • A damaged high limit switch
  • A faulty ignitor 

Regular maintenance and replacement of your air filter will prevent the most common of these problems from occurring. When working on your furnace, keep safety in mind and always take the necessary precautions when dealing with electricity or flames. 


  • Chris Hewitt

    Chris is a Texas-based freelance writer who loves the outdoors and working in his garage. When he's not enjoying the Texas sun, he can be found tinkering with all sorts of things in his workshop.

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