The flame on the gas furnace pilot light stays on even when the furnace isn't running to supply reliable heat, so you don't have to keep relighting it. However, you may find that you need to relight it, and it won't stay lit. Several issues could be causing the problem. Follow these steps to troubleshoot a gas furnace pilot light that won't stay lit.  

Prepare to Troubleshoot the Pilot

Gather an old toothbrush, fine-grit sandpaper, stiff wire brush, compressed air, a degreaser, and a needle. Find the gas shut-off valve, which is commonly on an incoming pipe.

Rotate the valve right to shut off the unit. Then let it cool five to ten minutes. Look for the pilot light in a box-shaped device near the burners, or consult your manual for location, and turn off the pilot. Also, inspect the area for drafts that could blow out the pilot, and block them.

Clean Furnace Pilot Debris

The pilot light tip is prone to get clogged with debris and soot, which inhibits gas flow  Use an old toothbrush to clean the tip, or detach the tip, and soak it in a degreaser if it is extremely clogged. 

Remove stubborn grime with a metal file or a stiff wire brush. Untwist the pilot tube, and scrub it with fine-grit sandpaper. Push clogs out of the tube using the needle. 

Repeat the process cleaning the pilot burner and inside the burner air hole. Buy compressed air from a hardware store, and spray it into the flame orifice. Replace damaged pilots.

Inspect the Thermocouple

The thermocouple is a safety feature that senses when the pilot is on or off from the amount of heat. A malfunctioned thermocouple may shut down the unit if it doesn't' detect heat, even though the pilot is lit.

This occurs primarily from a bent or dirty thermocouple, or the pilot isn't aimed toward it. Attempt to bend the thermocouple back into alignment, and clean it.

Adjust Flame Size

Some furnaces have flame adjusters, which control the size of the flame. The furnace won't light if the flame is too small. Check your manual to determine if your model has one and for the location. These normally never need adjusting, but they could get knocked out of place during cleaning.

The adjuster is commonly a small bolt or screw installed near the pilot light. Adjust the screw or bolt, restore the gas, and try the unit again. Keep adjusting the mechanism until you get the correct flame size to turn on the burners.

For more information, contact a local heating service.