How to Diagnose Furnace Problems & Why the 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 then 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.) Feel free to have a look at my article troubleshooting your furnace for some easy to understand, but a bit more in-depth, help with your issue.
First, I'll explain how to understand the light and identify the code it's giving you. Then, I'll give advice based on the type of code you've gotten. This article is general knowledge that applies to 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, and the latter of the two is certainly the more common. You wouldn't believe how often fuses can blow. I always keep a few 3 amp fuses around for this reason. They're especially good to have on hand if you're attempting your own repairs, you may just blow one of these and need one in a hurry. are the ones you'll need. These Bussman 3 amp fuses
Your Furnace Light Is On But Not Blinking?
If your furnace light is on 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 SeriesClick thumbnail to view full-size
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, you may need to know how to reset your furnace.
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 the unit will try to restart so you can watch to see what's 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 Howview quiz statistics
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's most likely that the ignitor itself is bad or that the flame sensor is either bad, or just dirty. This is a very common issue. So if you're getting a code like this, have a look at my article I wrote about what to do if 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 PartsClick thumbnail to view full-size
Furnace Codes for Pressure Switch and Inducer Failures
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 High Limit Switches
Codes referring to high 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.
High Limit Switches
The high 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.
Roll Out Switches
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.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Why would my furnace just start blowing out cold air? It doesn't try to light, the blower runs, and there's a small green light that flashes three times today.
The light is providing a code that corresponds with its code chart and says what part failed or failed to read what it is supposed to this narrowing down the possibilities for yourself or a service tech to pinpoint the issue. The blower comes on as a default feature upon failure. This is likely what has the blower running.Helpful 19
I have a g4ouh-48b-090-07 Lennox furnace -- My AC is working properly with the blower. My issues is that now that it is getting colder I need the heater to work. The board is lighting up, and I hear clicking and a slight hum but my ignitor is not getting hot, and my inducer motor is not turning on. I get an error code that says my pressure switch is the issue. My one LED is off, and my two LED is flashing slowly. What can I do? Is it actually the pressure switch or is the inducer motor bad?
From what you’re describing the inducer may be the issue since it’s not even starting. Perhaps this will help make the determination. https://hubpages.com/home-improvement/Everything-Y...Helpful 18
Why does my furnace blow cold air during the first 30 seconds of the heat cycle?
It may just be purging cold air that is in the ducts, kind of like when you turn on the hot water at the sink.Helpful 11
I have an older high efficient Frigidaire furnace. The red light flashes 4 times, shut power Off, sanded ground n reattached. Pushed both reset buttons . Power on and it cycles 4 times but won't light. No heat on igniter at any time. Would the igniter be shot or could something else not let it heat up?
Check the hot surface igniter for any signs of cracks often seen as white streaks. That would help determine if it’s bad. Getting one to try isn’t expensive and it doesn’t hurt to have a spare on hand since they often will go bad sooner or later and usually not at convenient times.Helpful 9
My Goodman furnace will light, but only stay on for a few seconds. I have cleaned the flame sensor and tried a different one. Everything else seems to work normal (exhaust and blower fan), and there are no codes (light is steady). Any suggestions?
I can't say for sure, but it could be a high limit/fan switch....maybe the board? Might try the limit since it's cheap and if that doesn't work call a tech.Helpful 6
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