If the pilot light on your gas-fueled home furnace won't stay lit, then it is likely the thermocouple has failed. Fortunately, testing the thermocouple is easy, and if it is necessary to replace it, the cost for a new thermocouple is often less than ten dollars. Below is more information on what thermocouples do, how you can test your thermocouple for proper functioning and install a new one, if necessary:

The purpose of the thermocouple on your furnace

A thermocouple is a simple, current-producing device that relies on a physical phenomenon known as the thermoelectric effect. When two dissimilar metals are joined together and heated, a tiny amount of electrical current is produced.

On your gas heating appliance, the thermocouple is used as a safety device to shut off the gas supply should the pilot light be extinguished. The current produced while the thermocouple is being heated by the pilot light flame keeps the gas valve open; if the pilot light quits burning, the thermocouple cools down and stops generating current, thus closing the valve. A faulty thermocouple will prevent the valve from opening and subsequently prevent the furnace from operating, as well.

Testing and replacing thermocouples - tools and materials needed

• Small adjustable wrench

• Digital multimeter

• Screwdriver set

• Propane torch

Procedure for testing and replacement of thermocouples

1. Turn off the electrical power and gas supply to the furnace - Before you begin performing any work on the furnace, take a moment to disconnect the electrical power and gas supply. Attempting to perform work on the furnace with the electricity or gas still connected could cause a devastating accident. The furnace will be connected via a breaker switch, and flipping this switch to the 'off' position will prevent a possible electrical shock. As for the gas supply, look for the shut-off valve that will be located close to the furnace, and turn the handle to the 'off' position.

2. Locate the thermocouple and remove it from the furnace - After safely disconnecting the power and gas to the furnace, find the sheet metal screws that hold the front panel in place and remove them. Carefully pull the panel away from the furnace and set it aside. On some furnaces, the panel is not attached by screws; rather, it simply is lifted up and away from the unit.

Once the panel is out of the way, look for the pilot light and thermocouple assembly. The thermocouple consists of a metallic rod attached to a thin copper tube which is screwed into the gas valve. Next, remove the thermocouple by unscrewing it from the gas valve with an adjustable wrench and removing any clips or screws that hold it in place next to the pilot light. Be careful not to crimp or pinch the thermocouple tubing during removal.

3. Test the thermocouple for proper functioning - After you have removed the thermocouple, position it on a flat surface for easy access. Next, attach a black lead from a digital multimeter to the tip of the electrical contact that was screwed into the gas valve; be sure the lead isn't touching anything other than the contact. The next step is to attach the red lead to the copper tubing approximately two-to-three inches above the electrical contact.

Set the digital multimeter to its direct current (DC) position, with the voltage range set to millivolts, which are thousandths of a volt. Note the panel should read zero. Next, apply flame from a propane torch to the thermocouple sensor, and monitor the panel reading to see if any readings appear. A functional thermocouple will generate at least 20 millivolts, while defective units will produce less than this or nothing at all.

4. Replace the thermocouple - If the thermocouple test indicate it is defective, then you will need to purchase a new unit from a heating supply store. To install the new thermocouple, simply screw it into the gas valve until it is hand-tight, then turn it one quarter of an inch further. Next, position the thermocouple sensor, so it is next to the pilot light, and reattach any screws or clips that held it in place.

For more information, or to hire a professional, contact a company like Glendale Heating & Air Conditioning.