Furnace Flashing Green Light

Is Your Furnace Flashing Green Light? Here’s What to Do

Furnaces heat homes and are essential for our comfort. When something is wrong with them, we need to manage the issue quickly and promptly. Manufacturers nowadays develop furnaces with straightforward diagnostic service with flashing red and green lights.

On coded systems

To get an accurate idea about how the furnace is doing, you need to catch its problem. You cannot fix the issue unless you know what it is. Manufacturers provide their own color-coded blinking light system to guide homeowners and HVAC technicians in discovering the state of a unit. Examining the lights and their actions will help you know the furnace’s status once you know the proper code.

How to find the code

Typically, you will find the manufacturer’s code in the manual and the furnace’s back. Sometimes, it’s attached to the circuit board so you can effortlessly spot it when in need. The lights might be attached to the circuit board and you need to look inside the furnace to find them. As part of the front panel, there are also furnaces with visible lights. The states for the lights are: off, on but not blinking, blinking steadily, and blinking in a pattern.

The patterns can vary

When the light is off, your furnace doesn’t have power. A plug may have become loose, or the circuit has tripped. The furnace is on if the light is steady but doesn’t generate heat. A constant and fast blinking light means the furnace works as it should and creates heat. Other patterns signal various issues that vary in severity.

The most common issues

It depends on the unit and the light could inform that the furnace has attempted too many times to start or won’t give heat unless it’s checked out. Gas valve issues, poorly located flames, and low flames are problems that need immediate servicing. You might pull from the wall and put it another way to solve electrical issues. When nothing works and the light doesn’t stop blinking, attention from professionals will be necessary.


While various furnace types and models have specific patterns, some gas furnace error codes are standard. Details come below.

What do the flashing lights signal

If the green light flashes rapidly, the furnace has a problem with the blower system, while it gives heat to the home. When the green light flashes slowly, the unit is on, but there is no commend for heat. A continuous light typically means that the IFC circuit board has to be replaced, whereas no light indicates that the furnace is off or doesn’t receive electrical power. Maybe it’s turned off for the season, the circuit has tripped, or a plug has become loose.

When the light is red and not blinking, the furnace has developed various problems that need further investigation. Also known as the “lockout light,” the red light shows that a safety mechanism on the furnace has determined a shutdown because it has identified a problem.

Turn off the furnace and call an HVAC technician if you notice a yellow light. It informs that the furnace is clogged with dirt and lacks oxygen. When the yellow light goes out, something is wrong with the furnace.

When the code informs on gas, flame, or an ignition failure, the issue comes from the burner assembly area (the flame sensor or the igniter) and requires inspection. You should always check the manual to see the error code. You can also get in touch with a certified HVAC technician so that he can make a complete diagnostic of the furnace and reset the circuit board to eliminate all the incorrect error codes.

Important note!

We advise you not to try to clear the error codes by resetting the circuit board. It would help if you had a certified HVAC technician investigate and perform repairs on the furnace when you see the lights flashing.

Green light flashing can inform on many things

As we’ve mentioned, various manufacturers will have the flashing green lights inform on specific situations. For instance, on a Lennox furnace, green light flashing is a routine reminder that the unit has finished the diagnostic self-check. Therefore, a flashing green light means that your Lennox furnace is performing as it should and there’s nothing for you to worry about. If the green light remains on or flashes on and off, the furnace has developed a problem.

A furnace has several states that it can be in at any given moment. It can be completely off, on but not being used, on and used for heating, and not working due to various problems. It’s tricky to differentiate between some of these conditions. Some manufacturers (Trane is one) displayed an easy-to-understand code in the blinking green light on the front panel. When you know the code, you can see what your furnace is doing at the time.

A flashing green light is a good sign

A green light flashing once and then going out commonly means that the air filter requires changing or cleaning. You can remove the filter and see if dirt and debris have accumulated. Call the professionals if red or yellow lights are blinking.

Sequence Flashing

When the light flashes in sequence, your furnace has problems requiring professional attention. The light will flash for the sequence, pause, and flash once again. You need to count the flashes and match them with the chart on the furnace’s back. You will diagnose what is the problem and give the professionals a heads-up. The light can flash between two and nine times.

Two, five, or six flashes

Two flashes indicate an external lockout due to repeated failures of the furnace to start. It’s a safety feature that stops the furnace from continuously trying to start when there’s a problem. If the green light flashes five times, a flame is present when it’s not supposed to be.

Green light flashing six times means that there’s a minor issue that you can also manage. You solve it by disconnecting the electrical plug from the outlet; you have to put it upside down from the way it was previously. If the light keeps flashing, the electrical problem is significant and you need to call the professionals.

Seven, eight, or nine flashes

When the green light flashes seven times, it means that there’s something wrong with the gas valve. It’s a problem that needs to be managed immediately. Eight flashes indicate the flames are low, whereas nine flashes mean the igniter needs checking. The igniter is used when the furnace has to start from a cold, so the problem isn’t severe. Nevertheless, when the furnace goes out and the igniter doesn’t work, the furnace will need repair to work again.

The conclusion

Even if green light flashing means the furnace is running as expected, you should check it out with the manual or the codes printed on the board. You don’t want to have the furnace break down in the middle of the winter.

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