Furnace Isn't Working? Check These Common Furnace Problems

Figuring Out Furnace Problems

- Furnace Troubleshooting - Common Furnace Repair Tips
- Video Guide - Diagnostic LED Introduction


A Furnace With No Heat? That's cold.

Are you getting cold feet? No, I'm not talking about taking vows. I mean has your furnace quit working and your feet are cold, literally? Well before you call out the pros, why not take a shot at fixing that furnace yourself?

There are a lot of things that can go wrong with a furnace but of those things, there are many that are easy to figure out and fix. They are also rather common so your odds of getting the heat back on sooner than later are actually not too bad.

Even if you need a part, don't be discouraged. Some of the most commonly replaced parts are easy to install and can usually be bought from a local supplier.

So, without further delay, let's see if we can't get those cold feet wet. I mean warm! Get your feet warm!

Quick Furnace Check List

  1. Check the thermostat
  2. Check for power - breaker, switch, indicator light, fuse
  3. Check for gas - inline gas valve is open
  4. Check filter
  5. Check for ignition - ignitor
  6. Check for ignition confirmation - flame sensor
  7. Check for flue/chimney blockage - pressure switch
  8. Know when to call a service technician

***Trying to fix a furnace yourself is fine but know when you need an HVAC professional. "Experimenting" can be dangerous and cause more problems than you already have.***

The following will help to explain the details of the above list.

Click thumbnail to view full-size
Check the thermostat outside first......then check the inside of the thermostat.
Check the thermostat outside first...
Check the thermostat outside first... | Source
...then check the inside of the thermostat.
...then check the inside of the thermostat. | Source

Check Thermostat Settings

I know it may sound silly but before we get carried away, let's check the thermostat for a couple of things. You might be surprised how much money per year is spent having guys like me come and turn up peoples' thermostat or replace the batteries in them.

The "How to Test A Thermostat" section will help if the below checklist has lead you to believe there are problems with your thermostat.

Thermostat Problem Checklist

It Might...
for a jumbled digital display
need replaced (we can test this to be sure)
for a flashing battery indicator
need new 'AA' batteries
temperature setting
not be turned up high enough
function selection
not be turned to "heat" (switch off and on again to be sure)
inside for loose or touching wires
just need tightened or tidied up
It may seem simple but sometimes big problems have easy fixes.

Mercury Controlled Thermostats Gotta Go

If you have the old tan, turn dial type thermostat that's temperature sensor is a mercury bulb, try to get it replaced as soon as possible. They're not nearly as accurate as newer digital models and since they contain mercury, they're not really safe either.

How to Test A Thermostat

Hopefully nothing on our checklist has lead you to believe the stat is bad but if it did...testing your thermostat is really easy to do and all you need is a medium to small sized screwdriver and a short piece of thermostat wire or similar. (This will only be a 24v circuit. Not "high voltage".)

Simply go to the furnace, remove the doors and follow these steps.

  • Locate the circuit board where the thermostat wires connect to the furnace.
  • Disconnect the wires connected to the "R" and "W" terminals. (the wires SHOULD be red and white too but aren't always,) Push them safely to the side.
  • Connect your small piece of "jumper wire" between the "R" and "W" terminals.
  • Put the door back on and see if the furnace works.

Did it work?

NO - Then your thermostat isn't the problem.

YES - Then it's likely your thermostat is bad and needs replaced. It could be the wires connecting the thermostat to the furnace but if you checked them for being loose, it's unlikely.

NOTE: Do not leave a "jumper wire" on the furnace to run it. It will make the furnace run continually without any temperature control which can be dangerous and cause other problems. This is only temporary and to see if the stat is bad.

Checking for Power to the Furnace

If you know the furnace is trying to light or hear the blower running but aren't getting any heat, then you can skip this. You have power. If not, then check the following...

  • Did a breaker trip?
  • Is the service switch at the furnace on? (Most units have this mounted to the furnace or ceiling just above.)
  • Is the little red light on the furnace's circuit board on? (Most modern furnaces have an indicator light to tell you it's getting power and if you're lucky, tell you what might be wrong. We'll get into that too.)

Got power?

Yes - Okay, then let's move on.

No - Then you may need an electrician or HVAC professional. Before you call though, let's check the fuse on the circuit board.

Did You Check the Filter? (I had to ask)

Though it doesn't cause as many problems for heat as it does A/C, did you check the furnace filter?

  • Yes
  • No
  • Uh, you're supposed to check that?
See results without voting

Checking if the Furnace Blew a Fuse

Most modern furnaces have a 3 amp fuses on the circuit board. Though it might be located differently on the board on different furnaces, it looks just like a standard size fuse you might find in your car and should be easy to find if it's there.

If you have an indicator light and it's not on or flashing, this may be your problem and is easy to determine and fix.

Simply pull the fuse off the board and visually look to see if the link inside is burned and/or broken. If so, you should be able to pick one up at the local hardware or automotive store and replace it. DO NOT substitute the fuse with one of another amp rating.

The Furnace's Red Light Is Your Friend

A flashing red furnace light can help you figure out your furnace problems.
A flashing red furnace light can help you figure out your furnace problems. | Source

Reading the Flashing Red Light on Your Furnace

If you have the little indicator light on your furnace's circuit board, usually visible through a sight glass on the furnace door, and it's flashing, it's trying to help.

This light blinks a sort of Morse Code to indicate a number that corresponds to a chart located somewhere on the furnace. Usually on the doors that you can remove from the front. For example, a would be the number 22 and on the chart may say something like, "ignition failure" or "pressure switch open".

Understanding the red blinking light on your furnace will help you navigate through the following possible issues that have a possibly simple solution.

Can you handle minor furnace repairs? This quiz can help you answer.

Furnace Pressure Switch Problems

Often a furnace problem indicated as being an open pressure switch has to do with a flue/chimney blockage. Now it's possible the pressure switch is bad but before we decide that, let's see if the chimney is blocked. Birds and other small animals have been known to take up residence in furnace flues when it's particularly cold. Check for this and or icing by:

  • Remove the chimney pipe from the top of the furnace to see if there's anything in there. If you have a high efficiency furnace, this may be difficult but animals are less likely for you. Instead, check outside to make sure the end of the pipes are clear and not iced over.
  • Make sure the little fan hooked to the flue (the inducer fan) is working. If not, you'll probably need a service tech.

DO NOT get up on the roof to check the chimney unless you are completely confident and comfortable in doing so. It's not worth the risk since usually the problem isn't up there. On occasion, birds will build a nest in there but if you haven't had problems so far this winter, it's not likely your problem. This is something that usually shows up early in the heating season.

Hot Surface Ignitor Photos

Click thumbnail to view full-size
This is what a hot surface ignitor looks like when it's working.Hot Surface Ignitor
This is what a hot surface ignitor looks like when it's working.
This is what a hot surface ignitor looks like when it's working. | Source
Hot Surface Ignitor
Hot Surface Ignitor | Source

Check for Furnace Ignition

This is easy. If you've come this far then watch and listen to your furnace as it begins to cycle. Just before you hear the gas valve open (click), you should see the hot surface ignitor begin to glow or hear the click, click, click of your spark ignitor. (You might even be able to see this spark.) If you don't see or hear these things happen, the furnace will stop the cycle.

If you're ignitor isn't working, the gas valve won't stay open until you replace the ignitor because the flame sensor is telling the valve that the gas isn't being lit.

Make sure the gas is on to your furnace. I have been on calls where kids playing around the furnace have turned knobs and flipped switches costing their parents grief and about $150 so be sure the gas is turned on as well.

If the ignitor is working but the gas valve didn't open, I am inclined to have you call a heating and cooling technician to work with a gas valve. This is not a simple repair and can cause dangerous issues if not done properly.

***If you have a standing pilot, just look to see that the pilot flame is lit. If not, this is likely your problem. Try to relight it just like you might a water heater. If it won't light then you probably need a new thermocoupler. These are cheap and can be bought at most hardware stores. (Measure how long of one you need.)

Check To Be Sure The Gas Is On

I've been on calls where kids playing around the furnace have turned knobs and flipped switches costing their parents grief and about $150.

Furnace Flame Keeps Going Out

If your furnace lights the burners and then shuts off just a few seconds later, this is almost surely a fault of the flame sensor. This is one of the most common furnace problems home owners face today.

Cleaning and/or replacing a flame sensor is one of the most common and simple heating and air conditioning repairs one can make. Without a doubt, you should give this a try on your own before calling an HVAC technician.

Writer's Qualifications

I have 18 yrs. experience in the HVAC industry, am EPA Certified and have a desire to help people in need.
I have 18 yrs. experience in the HVAC industry, am EPA Certified and have a desire to help people in need. | Source

Know When to Call an HVAC Professional

Beyond the items we've discussed above, there are still many things that could be wrong with your furnace and though you may not have wanted to, you should consider calling a professional HVAC technician. The simple fixes are usually identified at this point and messing around with much else could compound your problem and expenses.

Please understand that these tips are what I feel is fairly easy to check for and/or identify but anytime your working with a furnace, there is a certain amount of present danger that requires your attention and care. Never perform work on a furnace if you're not sure of what you're getting into and that you've taken all the safety precautions that you can with gas, electric and sharp edges. A career in HVAC requires knowledge of all these things.

I hope we were able to solve your problem but if not, at least you will have a much better understanding of your furnace which will help you with your upcoming service call. You'll be a very informed customer when they come knocking.

© 2014 Daniel Robbins

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Comments 10 comments

Christine Janz 5 months ago

Hi Daniel!

I have a 2010 Rheem 95% high efficienct furnace that is driving me crazy!!!

I thought I bought one of the best, but am now so frustrated bc the furnace will fire up, raise the temp about 2 degrees, and then the blower shuts off.

We have to turn the power switch off, let the furnace cool down, turn it back on an hour later just to repeat the same thing.

Our furnace repairmen have been to our house about 15 times in the last 3-4 months.

They were here yesterday. They cleaned the heat sensor - again - and left w furnace running for about 5 minutes.

Guess what happened next? It shut off.


Cre8tor profile image

Cre8tor 20 months ago from Ohio Author

Vincent - Sorry for the delay but...if you go to this link, about 50 seconds into the video, you'll see mine to the right of the inducer. I have 2 of them, you may not but those silver flat disk shaped parts are the pressure switches.

Vincent 20 months ago


I was wondering if there was a way to find the pressure switch on my Carrier 58MCA100-16. I have the manual and for the most part I can find all the parts listed. It shows my pressure switch behind the induced blower but there is nothing there. This may sound nuts but I simply can't find it. Would you be able to direct me?

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

@ Christy Kirwan - Thank you so much for your feedback! I'm glad to be able to help in my own little way. A sort of "pay it forward" if you will. Thanks again for stopping by and commenting.

Christy Kirwan profile image

Christy Kirwan 2 years ago from San Francisco

This is such a helpful guide! It's great of you to share your years of professional experience in a way that can help people save money. :)

mecheshier profile image

mecheshier 2 years ago

Me too :-) But I have had my fair share of gas stoves in the past. Talk about safety issues.

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

Thank you mecheshier! That's great to hear. I'm always concerned with formatting and you know, you ask yourself if it's "easy on the eyes" but who am I to say? Ha! Glad you liked it and hope you never have to use it!

mecheshier profile image

mecheshier 2 years ago

Great post. There are some very useful tips here. I love your format, it is easy to read and light on the eyes. I also had fun doing the quiz.

Voted up for useful and interesting

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

@sallieannluvslife - Thank you very much for the feedback. Though I hope you're furnace doesn't breakdown on you, I hope if it does, I can be of some help.

sallieannluvslife profile image

sallieannluvslife 2 years ago from Eastern Shore

Awesome Hub! Great advice to keep on hand just in case....

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    Daniel Robbins (Cre8tor)318 Followers
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    Dan has been in the HVAC industry for 22 years with experience in aspects ranging from installation and service to sales and distribution.

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