Furnace Repair: 11 Most Reported Problems with DIY Solutions (By Pro HVAC Technician)

The furnace is one of the essential parts of the AC unit, consisting of many different things. If your AC is not cooling or operating correctly, then there could be a problem with the furnace part. You need to know the most common issues with the furnace and how to troubleshoot and repair them.

So in this article, I’ve listed the top eleven most common furnace repair problems and how you can troubleshoot and repair them.

I've been in this HVAC business for over fifteen years, and over the past year, I have received many calls about furnace repair.

I know the most common problems with your furnace and how you can quickly repair them at home. But let me tell you one thing: some of the issues can be solved by DIY, but if the problem is significant, you have to call a technician.

Furnace Repair No #1: Dirty Flame Sensor - Most Reported Furnace Problems and Solutions

A flame sensor on a furnace detects and records the presence, duration, and intensity of flames to provide accurate information about the condition of the furnace. This information is used to set furnace temperatures and optimize combustion. If an abnormal flame is detected, the furnace will turn off its burner to prevent an uncontrolled fire from starting.

The number one problem that occurs in the furnace is a dirty flame sensor. From my clients, I often hear, “It seems like my furnace is working; I can hear it turn on.” When I hear burners coming on but only staying on for a few seconds before they shut off, I know that what they're experiencing is a dirty flame sensor.

A flame sensor is typically a thin, flat piece of metal that is suspended above the furnace. This metal heats up, and the sensor registers this heat as a signal. Most flame sensors are easy to locate and access, but sometimes they are very hard to find if you do not have the training of an HVAC technician.

To Access and Clean the Flame Sensor, Follow These Steps:

Step 1: Make sure you have disconnected or turned off the furnace power switch.

Step 2: Remove the furnace door, and at the bottom, you’ll find the flame rod behind the burner. You need to take it out with the help of a screwdriver. You can then determine if the flame sensor is dirty.

Step 3: Once there's enough dirt built up on your flame sensor, your furnace will stop sensing the flame. All you need to do is clean this sensor with any cloth or cleaning pad. You don't want to use sandpaper or any other abrasive substance on it.

After cleaning the flame sensor, you need to put it back in its place and close the furnace door.

A dirty flame sensor can reduce the accuracy of the furnace's temperature readings, potentially leading to poor heating or even fires. Additionally, a dirty flame sensor can also create a hazardous environment in the furnace, as harmful particles can be released into the air. By keeping your furnace clean and functioning at its best, you can avoid any potential problems.

Furnace Repair No #2: Dirty Furnace Filter - Most Reported Furnace Problems and Solutions

A widely reported furnace problem is a dirty furnace filter. A furnace filter is important because it helps to trap contaminants and dust particles that can be released into the air when the furnace is in use. These contaminants can cause respiratory issues and can also cause damage to your home's insulation and mechanical systems.

Dirty furnace filters can also cause problems for the mechanical health of a furnace. If somebody doesn't replace their filter for a long time, it'll get restricted, and the fan will start to suck debris into the furnace.

I've seen a couple of times where the filter gets completely sucked in and wrapped around the motor shaft; it's tough to pry it off. That's why one side of the filter has a net on it to prevent it from being sucked in.

Most of your filter is generally going to be on your return duct. Most people have it l-shaped; your filter will be in the return duct before the furnace. Just an FYI—your filters are directional; most of them will have an arrow that says which way the air flows. A furnace filter may not perform well if it is not faced the right way.

If you're like me and you tend to forget to change your filter, I started ordering my filters online. I get a whole box of them and change them from time to time. You should be changing them every month or two, and having a box standing by your furnace reminds you makes it is easier to swap them out.

Furnace Repair No #3: Bad Inducer Motor - Most Reported Furnace Problems and Solutions

So at number three, there is a bad inducer motor. Sometimes, the furnace won't function at all. When that happens, you descend and remove the furnace door.

If you put your hand on the inducer motor (be careful when doing so), it may be scorchingly hot, but there are times when it's not hot. In those cases, you want to check with your meter and see if it's actually getting 120 volts from the control board.

Because sometimes it's actually the board that's bad and not the inducer motor, where it's just not sending power to it. But long story short, if the inducer motor is getting 120 volts but it's not turning on, then the inducer motor is bad.

The only other scenario is if your motor has something stuck in it like a bird, a mouse, a bird’s nest, or something like that. An easy way to check is by removing the cooling fan in front of your inducer motor.

If you can access your fan, you can try to stick a little screwdriver in there and spin it to see if it spins. If it's spinning freely, I should say that means you probably don't have anything in there or anything stuck in there, and your inducer motor is just bad. And of course, the easy fix for that is to just replace the whole inducer motor.

Furnace Repair No #4: Bad Blower Motor - Most Reported Furnace Problems and Solutions

In the fourth place, there’s a bad blower motor. The blower motor is located at the bottom section of the up-flow furnace. At the bottom first, you’ll find the blower motor housing, and if you reach behind it, there's the blower motor.

There'll be a capacitor right below the housing; every blower motor will have a capacitor. If you reach in on the other side of the housing, you can put your hand in there and spin the blower wheel or the squirrel cage.

If the blower motor is bad or seizing up, you won't be able to freely spin that fan; you'll feel that it's seized up or that it's hard to spin.

Another indicator is that if you reach in there and touch that motor, it'll be really hot most of the time, and why that fails is because your filter has been dirty for a long time. Maybe there was just some kind of electrical surge; many times the capacitor is dead.

The capacitor gives the fan a boost to get it spinning; if that thing's completely dead, your fan will have a really hard time turning on. Some of the symptoms of a bad blower motor include the fan starting very sluggishly and then completely quitting, or smelling some kind of a burnt electrical smell in the house sometimes.

The only remedy for that is to replace the blower motor. Additionally, if the wheel is bad, replace the wheel, and you always replace the capacitor.

Check out How To Check A Bad Furnace Blower Motor (Step By Step Guide) for more information.

Furnace Repair No #5: Bad Control Board - Most Reported Furnace Problems and Solutions

Coming in fifth place is a bad control board, which will generally be right in front of the blower housing at the furnace. The Control board may look differently on various furnaces.

Older furnaces will not have control boards. They'll have a few relays, but most furnaces will have a control board. Sometimes, the furnace is not working or doing something weird, like the fan will not turn off no matter what you do.

You must turn the furnace switch off to turn the blower fan off. A lot of times, when you take the board off and look on the other side of it, you'll see some burn marks on it, and of course, that's a dead giveaway that the board is terrible. You’ll need to get it repaired or replaced by a technician.

Furnace Repair No #6: Bad Gas Valve - Most Reported Furnace Problems and Solutions

Coming in sixth place is a bad gas valve. Generally, the gas valve will be in the upper section of the furnace. If your gas valve is getting power, yet it's not opening, then that just means that the solenoid on the gas valve is bad.

And generally, you can't just replace the coil on it; you have to replace the whole gas valve. You can identify this easily if your furnace appears to run normally at first. Everything will run properly; your inducer motor will start, your pressure switches will close, and you'll see your igniter glowing. Then that's it; nothing else happens; it just glows and then shuts off.

Furnace Repair No #7: Bad Heat Exchanger - Most Reported Furnace Problems and Solutions

In seventh place is a bad heat exchanger. The heat exchanger is placed behind the wall of the furnace. So what is a heat exchanger, and why is it so important?

The heat exchanger is the main component of the furnace, and that is the thing that separates your house air from the combustion fumes that are going out of your chimney.

Usually, it's a tubular-looking thing that snakes back and forth. Now some circumstances can kill them a lot faster.

So, for example, the dirty filter that we talked about earlier, if your furnace is constantly overheating, will kill your heat exchanger a lot faster, or a plugged a-coil or inadequate ductwork, or maybe your furnace was installed poorly, to begin with.

These factors can decrease the lifetime of a furnace. We can tell if the heat exchanger is wrong by putting a combustion analyzer probe into the chimney or your exhaust pipe. And seeing how much carbon monoxide is going out your chimney.

It should generally be under 100 parts per million on a normally operating furnace. But if you have a terrible furnace or a bad heat exchanger, your carbon monoxide parts per million will go through the roof. They'll be like 2,000, 3,000, and 4,000.

Checking a lousy heat exchanger is so important because the heat exchanger is the thing that keeps all the combustion fumes out of your house.

So if that thing has a crack or a hole in it and it's producing that much carbon monoxide. And somehow, that carbon monoxide leaks into your air supply, especially at night when everybody's sleeping, it will only take minutes to kill everybody in the house.

Furnace Repair No #8:Bad Ignitor - Most Reported Furnace Problems and Solutions

In the eighth place, we have a bad ignitor. An igniter is like a plug that you can plug and unplug. If you turn the furnace switch off and turn it back on, you'll see the inducer motor come on after a while.

You will hear the gas valve open, and a little gas will come out, but the igniter is not glowing most of the time. That means your igniter is no longer functional, and you’ll need to replace it.

Check out How to Tell if Your Furnace Ignitor is Bad for more information.

Furnace Repair No #9: Plugged A-Coil - Most Reported Furnace Problems and Solutions

In the ninth place, we have a plugged A-coil. So it'll be a similar scenario where the furnace is overheating and tripping on that high limit.

If you have an up-flow furnace generally, the A-coil will be sitting on top of your furnace, and you'll see these refrigerant pipes come into it.

It gets plugged up with time since it's sucking air on the sides, especially if you don’t change your filter at regular intervals,

If you're not very good at changing your furnace filters or if you have a filter that lets a lot through after a while, all the dust and debris that gets past the filter will settle on the underside of that A-coil and plug it up, causing a restriction in the airflow.

So a lot of heat starts to build up in there because it can't get out, and that, of course, will trip your high limit once it gets too hot. You can hire duct cleaning technicians to do this job.

Furnace Repair No #10: Bad Ignition Module - Most Reported Furnace Problems and Solutions

In the tenth place, we have a bad ignition module. An ignition module is a control board, but the difference between a control board and an ignition module or ignition control is that the ignition module is only for the ignition.

So it only controls the gas valve and the spark if you have a spark igniter and a flame sensor, whereas this control board controls both the gas part and the fan, which is why sometimes it's called the IFC, which stands for integrated furnace control.

So sometimes what it'll be doing is, if it's a spark igniter, it won't be sending the spark to the igniter, or maybe it's not sending power to the gas valve.

But basically, you would check with your meter to see if it's sending power to the gas valve or if it's sending energy to the spark. And if it's not, the ignition module is wrong and must be replaced.

Furnace Repair No #11: Bad Thermostat - Most Reported Furnace Problems and Solutions

In the eleventh place, we have a bad thermostat. If you turn on your heat, your furnace, or even your fan, and nothing happens, maybe just one of them is not working.

Yet if you take the thermostat off the wall and jump the R-wire to the W-wire and everything works well, you know that just the thermostat is bad. Nothing's wrong with the furnace, but you will likely need a new thermostat.

Check out How to Bypass a Thermostat: A Complete Guide for more information.

Conclusion - Most Reported Furnace Problems and Solutions

The interior of the house may get extremely chilly when the furnace breaks. This may result in issues with the pipes that heat the home, making the interior uncomfortable. The cost of furnace repairs is another factor.

You should be able to work on your furnace just fine as long as you have the right tools and follow the proper safety procedures. You might even discover that repairing furnace troubles is preferable or more economical. However, think about hiring someone else if you don't feel confident servicing your furnace. It may be safer and more effective for a qualified professional to examine your heating system, especially for complex problems.