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Gas Furnace Not Lighting? Try Cleaning the Thermocouple

Inside your gas furnace lies a small, inexpensive electronic component called a thermocouple. This unassuming device plays a huge role in the functioning of your furnace, though, and will keep the entire system from operating if it fails to work.
If your natural gas furnace refuses to light this winter, your thermocouple may be dirty. Below is more information on this critical component and how you can clean the part yourself.
What Is a Thermocouple?
A thermocouple is nothing more than a temperature sensor, but its role in furnace operation is critical to preserving safety. When the furnace receives a signal from the thermostat to begin heating, the thermocouple will check for the presence of a pilot light before permitting the system to feed gas into the burners.
This pressure check is critical, as unburned gas could otherwise accumulate and create an explosion hazard inside the furnace and vicinity. However, a problem arises when the thermocouple malfunctions due to external soot buildup. At that point, the furnace will not light even if a pilot light is present and ready to ignite the burners.
While other problems may be the cause of a furnace that refuses to light, you should immediately suspect the thermocouple is the problem if the unit blower starts but immediately stops once the call for heat is made.
How Do You Clean a Thermocouple?
Fortunately, a thermocouple problem is usually far from serious and can be corrected easily, even by homeowners with an ability to use basic hand tools. Below is how you can clean a dirty thermocouple.
1. Disconnect Electrical Power and Natural Gas
Before you begin to perform any work on the furnace, disconnect electrical power and the natural gas supply to the unit. Any attempts to perform work without this crucial step could lead to electrical shock or even an explosion.
You can disconnect power to the furnace at the main circuit breaker panel, and the furnace should be served by a separate gas line. Locate the shut-off valve on the gas line and turn it 90-degrees until the valve handle is perpendicular to the line.
2. Locate the Gas Burners and Remove the Thermocouple
Once power and gas have been shut off, the next step is to locate the gas burners. This section of the furnace will usually be located behind a sheet metal enclosure, and you may have to remove a few screws to access it. Be careful when working with sheet metal to avoid cuts, and place the screws in a safe location for later replacement.
Once you have removed the cover to the gas burners, you will be able to view a small gas line leading to the pilot light assembly and a single electrical wire will run roughly parallel to the thermocouple. Follow the electrical wire until you locate the thermocouple, which consists of a thin, metal probe. The thermocouple is held in position with a screw and can be easily removed.
3. Clean and Replace the Thermocouple
After you remove the thermocouple from the burners, use an emery cloth to clean the tip of the probe. You can also wipe the probe with rubbing alcohol to remove oils or other greasy substances that might have accumulated.
Once the thermocouple is clean, reinsert it into position and fasten the screw holding it in place. Be sure the wire is still intact, and carefully reattach the sheet metal cover while avoiding pinching the wiring or damaging any other components.
4. Relight the Furnace
When you have cleaned and replaced the thermocouple, restore the electrical and gas supplies to the furnace. Next, consult the manufacturer's instructions for lighting the furnace, and enjoy the newfound warmth.
If you don't feel comfortable cleaning a dirty thermocouple or if your furnace still won't operate even after cleaning the thermocouple, be sure to contact After Hours Heating & Cooling, LLC for help. Our team of professionals is ready and able to assist you in restoring the warmth you've been missing.