Furnace Repair: 11 Most Reported Problems With DIY Solution (By Pro HVAC Technician)

Furnace Repair

The furnace is one of the most important parts of the AC unit and it consists of many different things. If your AC is not cooling or operating correctly then there can be a problem with the Furnace part. You need to know what are the most common problems that occur in the furnace and how to troubleshoot and repair them.

So In this article, I’ll list the top 11 most common Furnace repair problems and how you can troubleshoot and repair them.

I’ve been in this HVAC business for almost 6 years now and over the year I receive too many calls about furnace repair.

I know what are the most common problems that can occur in your furnace and how you can easily repair them at home. But let me tell you one thing some of the problems can be solved by DIY but if the problem is big you have to call a technician.

Furnace Repair No #1: Dirty Flame Sensor

Dirty flame sensor

So the number one problem that occurs in the furnace is a dirty flame sensor. So most of the time what people complain about is it seems like my furnace is working I can hear it turn on.

In fact, I can even hear my burners coming on but they only stay on for a few seconds and they shut right off. And that’s when I know that almost guaranteed what they’re experiencing is a dirty flame sensor and all that is is a stainless steel rod that will be behind the burners.

Most of the flame sensors are easy to access as but sometimes in some ac units it’s very hard to find if you’re not an HVAC technician. 

So 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: Then remove the furnace door and at the bottom, you’ll find the flame rod behind the burner. So now you need to take it out with the help of a screwdriver. And you can see if the flame sensor is dirty.

Step 3: So what really happens is once there’s enough dirt build up on it your furnace will stop sensing the flame. And all you need to do is clean this flame sensor with any cloth or Scotch Brite pad. Remember you don’t want to use sandpaper on it and you can also use the currency to clean it. And also there are some rumors going around that if you clean your flame sensor with a hundred dollar bill it’ll last a hundred times longer I haven’t tried that myself. You can give it a shot.

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

Furnace Repair No #2: Dirty Furnace Filter 

dirty furnace filter

So the second most-reported problem is a dirty furnace filter. If for some reason somebody doesn’t replace their filter for a long time it’ll get really restricted and the fan will start to suck it in.

And I’ve seen that a couple of times where the filter gets completely sucked in and wrapped around the motor shaft it’s really hard 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 like and your filter will be in the return duct right before the furnace. And just so you know your filters are directional most of them will have an arrow on them that says which way the air flows are.

So if you’re like me and you forget to change your filter a lot what I started doing is just ordering my filters on amazon I get a whole box of them and change them from time to time.

You should be changing them really every month or two and having a box standing by your furnace kind of reminds you makes it easier to just swap them out. 

Furnace Repair No #3: Bad Inducer Motor

So at number three, there is a bad inducer motor. What that will look like many times the furnace isn’t working at all you come down you take off the furnace door.

And if you put your hand on the inducer motor a lot of times it’ll be scorching hot you’ll like burn your hand or 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 or a mouse or some bird’s nest or something like that. And an easy way to check that is sometimes you’ll have a cooling fan in front of your inducer motor.

If you can access your fan you can try to stick like a little screwdriver in there and spin it and 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

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 is blower motor housing and if you reach behind it in 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 it’s hard to spin.

Another indicator is 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 was 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. So the symptoms of a bad blower motor are the fan starts very sluggishly and then completely quits or sometimes you’ll smell some kind of a burnt electrical smell in the house.

The only remedy for that is to replace the blower motor if the wheel is bad replace the wheel and you always replace the capacitor as well and if that’s the problem you’re experiencing at your house. 

Check Out Our Full Guide On How to Troubleshoot A Bad Blower Motor.

Furnace Repair No #5: Bad Control Board

Coming in the fifth place is a bad control board and the control board is generally going to 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 just have a few relays but most furnaces will have a control board. So sometimes what happens is either the furnace is not working at all or it’s doing something weird like the fan will not turn off no matter what you do.

You would have to turn the whole furnace switch off just 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 bad. You’ll need to get it repaired or replaced by a technician. 

Furnace Repair No #6: Bad Gas Valve

Coming in 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. So what that would look like is your furnace will start up normally.

Everything will run properly, your inducer motor will start, your pressure switches will close, you’ll see your igniter glowing and that’s it nothing else happens it just glows and then shuts off. 

Furnace Repair No #7: Bad Heat Exchanger

In the 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 basically the thing that separates your house air from the combustion fumes that are going out your chimney.

Usually, it’s a tubular-looking thing it kind of snakes back and forth. Now there are circumstances that can kill them a lot faster.

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

These all factors can decrease the lifetime of a furnace. We can tell if the heat exchanger is bad or not is by putting a combustion analyzer probe into the chimney or into your exhaust pipe. And seeing how much carbon monoxide is going out your chimney.

It should be generally under 100 parts per million on a normally operating furnace. But if you have a bad 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, 4,000.

Checking a bad 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 out 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

In the eighth place, we have a bad ignitor. So ignitor is like a plug that you can plug and unplug. So what it looks like is you turn the furnace switch off and turn it back on. And you’ll see the inducer motor come on after a while.

You will hear the gas valve opens and a little bit of gas will come out but the igniter was never glowing most of the time. That means your igniter is bad and you’ll need to replace it.

Check Our Full Guide on How To Check And Replace Ignitor

Furnace Repair No #9: Plugged A-Coil

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.

You know how it gets plugged up with time as it’s sucking air on the sides. And if you don’t change your filter at a regular time.

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

In the tenth place, we have a bad ignition module. An ignition module is basically 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 the 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 for some reason 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 just check with your meter to see if it’s sending power to the gas valve or if it’s sending power to the spark. And if it’s not then the ignition module is bad and needs to be replaced. 

Furnace Repair No #11: Bad Thermostat


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

Yet if you take the thermostat off the wall and jump R to W. Then the furnace turns on and everything works well. In that case, you know that just the thermostat is bad and nothing’s wrong with the furnace itself and you can just get it replaced.

You can check my full Guide on How to Check and Bypass The Thermostat.

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