report

How to Diagnose Furnace Problems & Why Red Light Is Blinking

Furnaces of today all come equipped with an LED light mounted to the circuit board. No, it's not there so you can find it in the dark. It's there to help you understand what your furnace is doing.

Whether it's working just fine or if it's completely kaput, this light is the voice of your furnace and if you don't speak its language, well, you're just not bonding with your unit as you should.

This article will explain how to read the light codes, interpret them, and understand what your furnace problem might be (should you have one.)

First, I'll explain how to understand the light and identify the code it's giving you. Then, I give advice based on the type of code you've gotten. This article is general knowledge for most furnace brands.

Reading and Interpreting the Flashing Red Light

Let's start with how to read this light and interpret what it means.

Surely you've heard of Morse Code. Well, your furnace uses a similar dash and dot system. Of course, it's not giving you the letters of words. Instead, it tells you a number that is a code for a specific type of problem that you can look up.

If it's a single digit, it will repeat the same dot pattern over and over with short pauses in between, like dot, dot, dot, pause, dot, dot, dot, pause. This would be a "3."

If it's a double digit number, it will use a dot, pause, dash, pause pattern to guide you on your way, like this: dot, dot, dot, pause, dash, dash, dash, pause, dot. . . you get the idea. This would be a "33."

What to Do With the Code

So now that you have the code, what do you do with it you ask?

Well, each furnace with this feature will have a code chart for you to refer to with your newfound knowledge. Most often this chart is found right on the furnace compartment doors for your convenience. For some, however, you may have to refer to your owner's manual. Here's a list of some major furnace brands and their codes.

If you can't find it anywhere you can check online, call a supplier, or even call the manufacturer directly. Have your make and model ready if doing so to speed up your search and ensure you get accurate information.

What Happens Next?

Now that you've gotten the code and referred to the troubleshooting chart for its meaning, it's time to see what the problem is. That takes a bit more knowledge.

You see, this light isn't a miracle worker. It's job is basically to point out the source of your furnace's problem. It's up to you to take it from there.

The following will help you know where to look and what you should be looking for when trying to pinpoint your furnace issue.

Your Furnace Light Isn't On at All?

If you know you have this light but it's not on, you are either not getting power to the unit (check your breakers and service switch) or you've blown the fuse on the circuit board.

The good news is that these are all simple furnace problems to fix. Keeping a few 3 amp fuses around is not only cheap but it's great in case of an emergency.

Your Furnace Light Is On But Not Blinking?

If your furnace light is and steady, this is a good thing. You're probably not reading this article in the first place, but if you are, carry on as normal.

Sample Code Chart for Carrier WeatherMaker 8000 Series

Click thumbnail to view full-size
Carrier WeatherMaker FurnaceFurnace code light and chart locationTop of Code ChartMiddle of Code ChartBottom of Code ChartRating Plate - Model Number and Serial Number are located here.Furnace Circuit Board where LED and fuse are located.
Carrier WeatherMaker Furnace
Carrier WeatherMaker Furnace | Source
Furnace code light and chart location
Furnace code light and chart location | Source
Top of Code Chart
Top of Code Chart | Source
Middle of Code Chart
Middle of Code Chart | Source
Bottom of Code Chart
Bottom of Code Chart | Source
Rating Plate - Model Number and Serial Number are located here.
Rating Plate - Model Number and Serial Number are located here. | Source
Furnace Circuit Board where LED and fuse are located.
Furnace Circuit Board where LED and fuse are located. | Source

See the tiny boxes at the top of this photo? You can click through those to see the pictures up close and personal. Though this might not be your own furnace's make or model, it'll give you an idea of what you should be looking for.

Codes that Confirm Normal Furnace Operation

Some people will be confused by the "blower running after power up" code or something similar. If you're getting this code, it actually means that things are okay.

If your furnace has just been installed, or if you've just lost power or cut the power to your furnace, the blower will fire up once the power is introduced.

It does this as a sort of system check or stretching exercise before it really gets to work. Give it a minute or two. It will stop and do whatever your thermostat is telling it to do following its warm up.

That reminds me . . . . . . . . . . . . . . .

How to Reset Your Furnace

You can reset your furnace by cutting the power to the unit via the service switch or the breaker. Leave it off for about 30 seconds and then turn it back on. This will clear the board and let the unit try to restart so you can watch to see what is happening.

Be sure to get the code the furnace is giving you before you cut the power. This code will be lost once the unit is powered down. Sometimes, if you're lucky, the unit will run after you've restarted it. Not that it's fixed but if luck leans your way, you might have heat again, at least for the time being.

It happens more often than you might think!

Let's Test Your HVAC Know How

Furnace Codes for Gas, Ignition Failure, or Proving Fault

Any time you get a code referring to gas or ignition failure, or flame proving failure, your problem will exist in the burner assembly area.

It is most likely that the ignitor itself is bad or that the flame sensor is bad or dirty. However, there are times where the gas valve is bad or malfunctioning. At least now you know to watch in that area to see what exactly is happening the next time it tries to start.

It's pretty easy to detect and repair ignitors as well as flame sensors. Gas valve problems, however, are above most homeowner's experience level and a professional should probably be called.

Burner Assembly and Parts

Click thumbnail to view full-size
Burner Assembly AreaHot Surface Ignitor ExampleFlame Sensor ExampleGas Valve ExampleReference Furnace
Burner Assembly Area
Burner Assembly Area | Source
Hot Surface Ignitor Example
Hot Surface Ignitor Example | Source
Flame Sensor Example
Flame Sensor Example | Source
Gas Valve Example
Gas Valve Example | Source
Reference Furnace
Reference Furnace | Source

Furnace Codes for Pressure Switch and Inducer Failures

Inducer Fan/Motor (bottom left) Pressure Switch (right of inducer) and Chimney Connection/Inducer Housing (top center)
Inducer Fan/Motor (bottom left) Pressure Switch (right of inducer) and Chimney Connection/Inducer Housing (top center) | Source

Whenever you get a code referring to pressure- and inducer- related issues, they have to do with the area of your furnace shown above.

This area contains pressure switches (which usually look like little flying saucers), the inducer motor/fan (used to help vent unwanted gas byproduct out of the chimney or the flue), and the chimney connection to the inducer housing.

Though most problems here will require a professional, there are a few things you can check like:

  • Is there a nest blocking the chimney?
  • Is there ice blocking the chimney?
  • Is there an animal caught in inducer housing?
  • Is the inducer fan working?

Furnace Codes for Roll Out (FRS) and Limit Switches

Codes referring to limit or roll out switches and sensors are talking about these little guys. They are located in various places on the furnace and typically detect overheating or heat in places where it shouldn't be.

Limit Switches

Fan Limit or High Limit Switch: These can not be reset in newer units however the older models and switches could.
Fan Limit or High Limit Switch: These can not be reset in newer units however the older models and switches could. | Source

The limit switch is used to detect temperature inside the heat exchanger compartment. It will communicate with the fan to tell it when to turn on and off but it also acts as a safety in case the exchanger cabinet gets too hot. There is only one of these.

Roll Out Switches

Roll out switches are more for where heat shouldn't be. For example, if the flames in the furnace were to shoot out the wrong way or back flash, the roll out switch would detect that and shut down the unit.

In other places, like mounted to the side of the blower, the roll out switch would detect a fan motor overheating and shut it down as well. This could be from a dirty filter, lack of air movement caused by a deteriorating motor or a motor that isn't running, or because it's bad or the capacitor it uses is bad.

A capacitor just gives a "kick start" to the motor when it turns on and resembles a small tin can or 9V battery. If the top of it is popped up, that's likely your problem. Your air conditioner uses the same type of thing.

What Does Furnace Lockout Mean?

There is a code for "lockout" on most units too. This means just what it sounds like. Everything is locked out for use.

Lockout occurs when a furnace has attempted to fire multiple times (usually three) and failed. It will run the blower for a few seconds as a default and then shut down completely. Above, I explained how this can be overridden and how to handle the situation if needed.

Doing Work on Your Furnace Can Be Costly

I hope this information has been helpful to you but remember, working on a furnace can be dangerous and if you aren't sure about what you're doing, not only can you get hurt but you can cause further damage to the furnace.

It's never a good thing when you have to call a professional HVAC technician and explain how you tried to fix your furnace and now it's in pieces. Even if it's not in pieces, it's usually pretty easy for a technician to see when someone has been tinkering with things out of their wheelhouse.

Always put your safety first. Enjoy fixing the things that you can but know when to call it a day and let someone else handle it.

© 2014 Daniel Robbins

More by this Author


Comments 7 comments

Bea 44 hours ago

My furnace blinks with a code 43. What does that mean and what can I do


Cre8tor profile image

Cre8tor 6 weeks ago from Ohio Author

Ninfa - It sounds like it's not lighting the gas if the blower is coming on that fast. Watch the unit as it tries to fire and see if the ignitor is bad. Thank you for reading.


Ninfa 6 weeks ago

My blower come on before any heat and i have a red light blinking


Cre8tor profile image

Cre8tor 8 weeks ago from Ohio Author

Steve - Look on the doors for a code chart or the owner manual. That will tell you what code 12 is. Fans run by default when there is a malfunction often times so that doesn't surprise me. It's possible the board is shorted out though since it seems erratic. Thanks for reading.


Steve 8 weeks ago

I have a code "12" blinking. Unit worked fine the other evening, then nothing. Blower was running non-stop, but no heat. Even continued to run when panels were removed. That, finally stopped. Suggestions?


Cre8tor profile image

Cre8tor 19 months ago from Ohio Author

Rich - If you have all the power to the unit correct/functioning then perhaps whatever blew that fuse shorted the board or the transformer. A 120/24v transformer is pretty cheap so you could give it a shot and then maybe consider a board.


Rich 19 months ago

My furnace was blinking 24 (aux fuse), so I replace the 3 amp fuse on the board, but now there is no red light at all-

Tried cutting powered at the breaker for a min, but still no luck, any ideas?

    Sign in or sign up and post using a HubPages Network account.

    0 of 8192 characters used
    Post Comment

    No HTML is allowed in comments, but URLs will be hyperlinked. Comments are not for promoting your articles or other sites.


    Cre8tor profile image

    Daniel Robbins (Cre8tor)317 Followers
    50 Articles

    Dan has been in the HVAC industry for 22 years with experience in aspects ranging from installation and service to sales and distribution.



    Click to Rate This Article