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How to Diagnose Furnace Problems & Why the Red Light Is Blinking

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Dan has been in the HVAC industry for 23 years with experience ranging from installation and service to sales and distribution.

This article will break down why that red light on your furnace is flashing and what you can do about it.

This article will break down why that red light on your furnace is flashing and what you can do about it.

The Voice of Your Furnace

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. If you don't speak its language, 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. Its 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.

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Read More From Dengarden

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. These Bussman 3 amp fuses are the ones you'll need.

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.