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Troubleshooting Common Furnace Problems With an HVAC Expert

Updated on January 19, 2017
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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.

Figuring Out Furnace Problems

Before you call out the pros, why not take a shot at fixing that furnace yourself?

I have been in the HVAC industry for over 22 years. There are a lot of things that can go wrong with a furnace, but many issues are easy for anyone to figure out and fix. Even if you need a part, don't be discouraged—many of the most commonly replaced parts are easy to install and can usually be bought from a local supplier. Your odds for getting the heat back on sooner rather than later are actually not too bad. Below, you'll find...

  • A furnace troubleshooting checklist.
  • Tips on common furnace repairs.
  • A video guide to help you fix it yourself.

Furnace Troubleshooting Checklist

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

Below, I explain each of these processes in detail. We start with the simplest problems and then test for more difficult issues. It's important to be methodical and systematic in your diagnosis.

#1: Check the Thermostat

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' thermostats or replace their batteries.

Thermostat Problem Checklist

It Might...
for a jumbled digital display
need replacing (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
Sometimes big problems have easy fixes.

Mercury-Controlled Thermostats Gotta Go

If you have the old tan, turn dial type thermostat that senses the temperature with a mercury bulb, get it replaced as soon as possible. It's not nearly as accurate as newer digital models and since it contains mercury, it's not safe.

How to Test a Thermostat

Testing your thermostat is really easy. All you need is a small or medium sized screwdriver and a short (6") piece of 18 gauge wire (what we'll call a "jumper wire"). If you go to the hardware store, ask for "18/2."

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

  1. Locate the circuit board where the thermostat wires connect to the furnace.
  2. Disconnect the wires connected to the "R" and "W" terminals. (The wires should be red and white, but aren't always.) Push them safely to the side.
  3. Connect your jumper wire between the "R" and "W" terminals.
  4. Re-secure the door 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 replacing. It could be the wires connecting the thermostat to the furnace, but they aren't loose, it's unlikely.

NOTE: Do not leave the jumper wire on the furnace to run it. It will make the furnace run continually without any temperature control which can be extremely dangerous and cause other problems. Only use the jumper wire to see if the thermostat is faulty.

ALSO: These instructions of for testing a regular 24v (low voltage) circuit but not a high voltage one. In other words, don't use these instructions if you're not troubleshooting your furnace.

#2: Check for Power to the Furnace

If you hear the furnace trying to light (ignite) or hear the blower running but aren't getting any heat, then you can skip this step, because 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.)

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 the Furnace Blow a Fuse?

Most modern furnaces have 3 amp fuses on the circuit board (although it might be located elsewhere on the boards of certain furnaces). It looks just like the standard size fuse you find in your car and should be easy to find.

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 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. DO NOT try to substitute the fuse with one of another amp rating.

#3: Make 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 a lot of grief (and about $150).

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?

See results

#4: Check the Flashing Red Light

The Furnace's Red Light Is Your Friend: A flashing red light helps you figure out the problem.
The Furnace's Red Light Is Your Friend: A flashing red light helps you figure out the problem. | Source

If there's a little indicator light on your furnace's circuit board (usually visible through a tiny window on the furnace door) and it's flashing, it's trying to help.

This light blinks a sort of Morse code, a number that corresponds to a chart located somewhere on the furnace. It's usually on the doors that you remove from the front. For example, a "dot...dot...pause...dot...dot...pause" would be the number 2, which the chart may say indicates 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.

#5: Check for Blockages

Furnace Pressure Switch Problems

Sometimes, the little red light indicates an open pressure switch, which often means a flue/chimney blockage. It's also 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 flues when it's particularly cold. Check for this (and/or icing) by...

  • removing 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. Also check outside to make sure the ends of the pipes are clear and not iced over.
  • making 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 the chimney, but if you haven't had problems so far this winter, it's not likely your problem. This is something that becomes apparent early in the heating season.

#6: Check Furnace Ignition

This is easy:

  • Watch and listen to your furnace as it begins to cycle. Just before you hear the gas valve click open, you should see the hot surface ignitor begin to glow or hear the click, click, click of your spark ignitor. (You might even see this spark.) If you don't see or hear these things, the furnace will stop the cycle.
  • If your 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. 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 pilot light, look to see that its flame is lit. If not, this is likely your problem. Try to relight it just like you would 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 before you go.)

A Furnace Flame That 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 homeowners face.

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.

#7: Know When to Call an HVAC Professional

Beyond the items we've discussed, there are many other things that could be wrong with your furnace. I have identified the simple fixes above, and if you still haven't diagnosed the problem, messing around any more could compound both the problem and the expense. Although you may not want to, you should consider calling a professional HVAC technician.

Please understand that any time you work with a furnace, there is a certain amount of 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.

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.

Trying to fix a furnace yourself is great, but know when you need an HVAC professional.

Experimenting can be dangerous and cause more problems than you already have.

© 2014 Dan Robbins


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    • Cre8tor profile image

      Dan Robbins 2 months ago from Ohio

      All those could be problems however if nothing works then I'd be inclined to suspect the board is the issue. That said, I can't be sure from here but I would be thinking it. Thank you for reading.

    • profile image

      I need help! 2 months ago

      Power to board but nothing comes on indoor unit nor outdoor unit. Can it be transformer or thermostat?

    • profile image

      Matt 3 months ago

      So my pilot ignites, but the furnace will not ignite from the pilot unless I fan it. it then comes on no problem. Once the furnace tunrs on and heat is blowing, it will shut down 1-2 times and then comes back on no problem. I think this may be 2 problems. Any thoughts?

    • Cre8tor profile image

      Dan Robbins 3 months ago from Ohio

      There would be no need or benefit for you to turn anything off at the furnace in the summer unless you have a standing pilot (which means the pilot light is always burning) which most modern furnaces do not have. If you do, then yes, shutting off the gas isn't a bad idea to save energy. Just remember you do it and be sure to get it lit up prior to the cold days not just so you're not cold but to make sure everything is working proper before you need it...not when. That's what leads to after hour service calls and their not so friendly costs. Thanks for reading.

    • profile image

      A Grover 3 months ago

      Great article. A quick question.

      I want to switch off the furnace for the summers. I switched off the gas, and the power switch on the furnace. Now the issue is that with the power switch off, my air conditioning does not work either. Is it okay if I turn the power switch on, without turning the gas on?

    • Cre8tor profile image

      Dan Robbins 4 months ago from Ohio

      It sounds like you may have had a short in the stat. That's a guess but you may just need to replace it. If you disconnect the stats wiring and the fan shuts off, that's a good sign that the stat is your problem. Thanks for reading.

    • profile image

      David Matos 4 months ago

      The fan on my furnace runs continually and I can't control

      because the thermostat is completely blank no function but I

      replaced the batteries but no effect.

      Please what can I do?

    • profile image

      abdul 6 months ago

      Thanks a lot....good information.

    • Cre8tor profile image

      Dan Robbins 8 months ago from Ohio

      Norman Brand - I'm sorry but I can't really advise based on this info. Certainly if you have to rap on the board something is up. Loose connection? Intermittent "brown" power? Perhaps try documenting conditions and times when this happens and see if you might not be able find a commonality or at least provide the info to a tech who may be able to find one.

    • profile image

      Norman Brand 8 months ago

      My thermostat will tell the furnace to come on but more and more it sits there and does nothing. If I open the service door and tap on the circuit board, it comes on and no problem. Do you think it is a problem with this board? Once it is on it works great, then a few days to weeks later, it does it again. LED is steady on. Thanks nrbrand@q.com

    • Cre8tor profile image

      Dan Robbins 8 months ago from Ohio

      Dave in CA - Thanks for reading and offering more input. Anything to help people get their equipment going is always welcome.

    • profile image

      Dave in California 8 months ago

      My furnace gave flash codes indicating that the Pressure Switch was Stuck Open. The inducer fan was turning just fine. I jumpered the two wires leading to the pressure switch, and the furnace ignited and ran fine, until it went off and then wouldn't come on again, because the "Pressure Switch was Stuck Closed", thanks to me. I removed the jumper wire.

      I disconnected the power. I removed and checked the chimney stack and found nothing. I checked the pressure switch by pulling one end of the little hose off the fan, and breathing in the free end of the hose, this made the switch open and close. I even looked up on top of the roof, the chimney was not iced over, blocked, or occupied by an animal.

      Finally, I tried hooking the little hose back up to the inducer fan, and breathing in the end that was attached to the pressure switch. I couldn't. I unbolted the inducer fan and opened it up to check inside for debris.

      Turns out the problem was that the hole inside the fan that hooks up to the hose was blocked by dust and/or corrosion. I poked it out with a paper clip and stirred it around, then tried breathing through the hose. Now I could get air in and out of the fan. I reassembled everything, including the wires to the pressure switch, and turned the power back on.

      Voila. The furnace ran perfectly.

      For your pressure switch section, I suggest a step (for older furnaces) to check to check if one can blow air in and out of the fan through the hose. If not, either the hose or the fan hole is plugged.

    • Cre8tor profile image

      Dan Robbins 10 months ago from Ohio

      Dpreem - I can only guess that the ignitor isn't getting the correct power which would have me think the board is bad. Can't guarantee that but would be my next suspect. Thanks for reading.

    • profile image

      Dpreem 10 months ago

      I have a GMPO 50-3 goodman heater. Board will cycle for about 30 secs then cut out. Steady red light on board. Cked pressure switch with suction on hose-works. Took hose off to ck and 3 blinks on board told me pressure switch open. Put hose back on back to steady blink. Changed electronic ignitor today. Flame sensor looks ok no visible problems Getting gas to unit. Took off blower inducer fan impeller looks good. Can not get unit to show a glow or get close to ignition. Is it possible blower inducer not working properly? Had a tech come out and cked most everything above said it had to be elec ignitor but said it was an easy fix, two screws and a plug. Did that today and still nothing, What am I missing?

    • Cre8tor profile image

      Dan Robbins 2 years ago from Ohio

      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.


    • profile image

      Vincent 2 years 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

      Dan Robbins 3 years ago from Ohio

      @ 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 3 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 3 years ago

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

    • Cre8tor profile image

      Dan Robbins 3 years ago from Ohio

      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 3 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

      Dan Robbins 3 years ago from Ohio

      @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 3 years ago from Eastern Shore

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