The furnace thermocouple is a key element to keeping your heater running. This tiny piece heats up from the flame of your pilot light, supplying an electrical signal to the gas valve, keeping it open. This safety feature ensures that the pilot is lit before allowing the gas to flow. Dirty thermocouples may not read the heat of your pilot efficiently, causing the gas supply to be cut off.
Turn Off the Power and Gas
Turn your thermostat to the "Off" position. Unplug power to the heater or throw the breaker that connects it. You should see it labeled in your breaker box. If not, turn the heater fan on and flip breakers until it goes off.
Locate the heater's pilot light assembly. It is typically on the front of the furnace and should be easy to access. It looks like a gray/silver metal block with several pipes and hoses connecting to it.
Open the metal cover on the heater above the pilot assembly to expose the pilot light. It looks like a bent tube with the side of the end cut out. The cylindrical rod in the flame or, if the flame is out, directly in front of the open end of the cut out tube, is the thermocouple. Locate and turn the knob on the top of the pilot assembly to the "Off" position. Wait for the pilot to go out if it is burning. It should go out almost immediately.
Trace the gasoline back from the pilot assembly to find the stopcock that cuts off the gas supply. It is typically a red knob or lever-type handle. When the lever or knob is in line with the pipe, it is open. Move it so that it is perpendicular to the pipe; it should turn clockwise. Continue turning until it stops.
Clean and Reset
Allow five minutes for any gas to clear before using your pliers to twist the nut at the back of the thermocouple to remove it. Pull it free from its bracket and examine it in good light. If it has a sooty surface, cleaning may do some good.
Use an ultra-fine steel wool or 250-grit sandpaper to clean the outside surface of the thermocouple. This can be done while the copper coil, that leads from its back, is still attached. Do not use any type of cleaning solution, chemicals or solvents. Many are flammable.
Replace the thermocouple, making sure to line it up in the same position it was in originally. Tighten the nut. Turn the stopcock until it lines up with the pipe and restore electricity to the heater. Turn the pilot light assembly knob to the "Light" position. With a lit long fireplace match or long butane match lighter positioned in front of the pilot light tube, depress the pilot light knob to release the gas. If it lights, turn the knob to the "On" position and replace the metal access plate. If it stays lit, set your thermostat to "Heat" and set the correct temperature.
If these steps do not work or you are unsure of what to do, call for professional help. Gas can be flammable and explosive.