On a typical gas furnace, the pilot light is a small gas flame that acts both as an ignition source for a larger burner within the furnace, as well as an indicator of whether your furnace is actually burning and producing heat.
A pilot light going out can be inconvenient because it will completely cripple your furnace. The gas may be the fuel, but the pilot flame is the ignition and one that won't re-light on its own. For better or worse, a pilot light extinguishing isn't all that uncommon as even a light breeze or draft of air is enough to kill the tiny flame. Since this is such a frequent issue, the procedure to fix it, or relight your pilot, is fairly easy to learn.
Step 1 – Locate the Pilot Light Assembly and Reset Switch
The pilot light is typically in the bottom of the furnace near a small knob. In most cases, this knob has three distinct settings: "Pilot," "Off," and "On." If you are unable to locate this assembly or knob, refer to the user manual for your furnace and try to locate a diagram that can tell you where the knob or switch is on your specific unit.
Step 2 - Turn the Knob to the "Off" Setting
You should be able to turn the knob with your fingers. Turn it so that the "Off" setting is selected.
Step 3 - Wait
Wait a little bit before doing anything else. This is a very small but very important step. Turning the knob to "Off" simply stopped the incoming flow of additional gas, but it didn't magically vacate any of the gas that was already emitted from the pilot and is still present. Wait and allow this gas to dissipate before you attempt to light a flame to reignite the pilot.
You only need to take a minute or two, but again, striking up a flame in a room that up until a moment ago had unburned gas being piped into it is a bad idea.
Step 4 – Light the Pilot Light
Turn the knob to the "Pilot" setting, which will re-start the flow of gas to the pilot. Strike up your match or lighter. Depress the reset button, and while it is depressed, hold the match flame near the pilot opening. When the pilot is burning brightly, release the rest button. The furnace will then start, automatically.
Step 5 - Observe
At this point you're done. The pilot is lit and the furnace is burning and should start generating heat shortly. Stick around for a moment and be sure that the pilot flame stays lit. Look around and inspect the area for any drafts or sources of air that could've blew out the pilot in the first place.
If you locate anything that seems like it will blow out the pilot flame again, address it. Otherwise you'll be right back where you are very soon.