Troubleshooting Furnace Issues

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Troubleshooting furnace issues is a skill any homeowner should possess. Like many appliances, furnaces often have built-in alert systems to notify you of a problem. Knowing how to interpret these signals can help you address the issue quickly. But sometimes it takes a little investigative work to determine what's wrong with a malfunctioning furnace – and to decide how to fix it.

Troubleshooting Furnace Issues
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Reading the Owner's Manual Is Key

No universal error light code exists for furnaces. If your furnace displays a flashing light or if a light that's normally green changes to orange, yellow or red, it could indicate a problem with the flame, pressure or fuse. To know exactly what the light means, you'll need to read the owner's manual for your specific furnace model. Most owner's manuals are available online if you've misplaced the original hard copy.

Smell of Gas Coming From the Furnace

If you suspect gas is leaking from your furnace, shut off the furnace, open the windows to ventilate your home and call a furnace repair company. If the gas seems to be leaking from the main gas valves, call 911 to report a gas leak and evacuate your home. Gas leaks can be fatal, either causing an explosion or killing you slowly via carbon monoxide poisoning.

Just because you don't smell gas coming from your furnace doesn't mean there isn't a leak. Carbon monoxide is odorless. Consider installing a carbon monoxide detector to keep you and your family safe.

Furnace Doesn't Blow Hot Air

If your furnace seems to only emit lukewarm or even cold air, multiple causes may be at play. Check your filter to be sure that it is not too dirty, is the correct size and is installed properly. All of these factors can cause restricted air flow.

A faulty thermocouple or flame sensor can also cause a furnace to fail to heat. The burners won't ignite if either of these parts malfunction. If the fan for your furnace is blowing but you cannot see any flames within the unit, the problem most likely lies in the thermocouple or flame sensor. They should be replaced.

A quick and simple fix might also solve the problem: Check the thermostat to see whether the heat setting says "on" or "auto." If the thermostat says "on," the fan may run even when the furnace is not burning. This will give the impression that warm or cold air is blowing through the vents. In the "auto" setting, the fan only runs when the furnace is ignited.

Finally, the air ducts might leak, causing the air to dissipate before reaching its final destination: The rooms in your home. The leaks can happen anywhere in the duct system, but a professional should be able to detect the source of the problem.


Cathy Habas enjoys distilling even the most complicated home improvement tasks into bite-sized pieces. She believes in empowering homeowners one article at a time.

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