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Furnace Blowing Cold Air? It May Be a Bad Thermocouple

When your furnace thermocouple becomes faulty, you can experience anything from your furnace pilot light going out to your furnace blowing cold air. These symptoms of a bad thermocouple can compromise your home comfort—something no one wants! So let’s get to the bottom of it.

What Causes a Thermocouple To Go Bad?

Your furnace’s thermocouple acts as a safety device, sensing the presence of the pilot light in your system. If this device senses the pilot light has gone out, it will enact its safety precautions and block the gas valve from opening. However, if your thermocouple has gone bad, it may sense the pilot light incorrectly and therefore may release unburned gas into your home.

Because thermocouples are constantly exposed to that pilot light flame, they can wear out over time and can lead to the part going bad.

How to Tell If Your Thermocouple Is Bad

  • Make sure that the thermostat is set to heating and that the setpoint temperature is 5℉ above the current room temperature. If you have a programmable thermostat, the desired temperature may have changed to a different setting accidentally and turned the furnace off. Though it may sound basic, always check the thermostat settings first.

  • Start by setting your thermostat to "off". Next, turn the power off to the furnace through either the circuit breaker or emergency shut off switch on or near the furnace.

    In most cases a front cover must be removed to view the pilot flame assembly. For a gas furnace it would be the upper door panel. A gas boiler may only have one front door panel. You will observe the pilot flame often located in the center just above the burners.

    Once you remove the panel, you'll see the pilot flame through the small window. A healthy pilot light will appear as a blue flame approximately 1" high. If you don't see the flame, then the pilot must be relit by following the manufacturer's "lighting instructions" posted on a label on the furnace or door panel. If you can't relight the pilot light, then the system either doesn't have gas or there's an issue with the thermocouple.

    CAUTION: Set the thermostat to off and disconnect the furnace's electrical power before removing any access panels.

  • If you have a natural gas furnace, make sure that your gas hasn't been shut off. Check the gas shut-off valves near the furnace and near the gas meter. If they're both open, check with the utility company to make sure that they haven't shut the lines for maintenance.

    If you have a propane gas system, the propane is supplied from either an above ground or inground tank that must be refilled by a propane delivery service. An above ground tank will have a hinged cover on the top where you can view a gauge displaying the fuel level in the tank. An inground tank will have a hinged cover, usually flat, to allow access to the gauge on the tank. Call your supplier for a fuel delivery if the gauge needle is in the red area or 0–20 range.

  • If your gas is on, the furnace should be running, and the pilot light won't stay lit, it's likely an issue with the thermocouple. Call a technician to come out and investigate further.

How to Fix a Bad Thermocouple

If you’ve determined the thermocouple is the source of your furnace issues, you’ll likely need to have your bad thermocouple replaced. You’ll need to have the system inspected by a technician to decide what your next best move is.

You can reach out to one of our Remote Assist experts for a free virtual diagnosis to get started!

NOTE: This content is for informational purposes only, and HomeX and its affiliates disclaim all liability related to it. If you decide to perform any tasks based on this information, you assume all risks, including the risk of loss or damage to property or personal injury.

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