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A thermocouple is a bi-metallic junction that when heated gives an small electric output. The tip of it sits where the pilot light is ( and it is a copper tube going to the gas valve) and the pilot light heats it up. It creates electricity after it heats up (which is why you have to hold the pilot light knob down when lighting) and opens the gas valve to keep pilot light lit after you let the pilot knob go back up. When it stays lit without having to hold it down, (make sure the thermostat is set low so it doesn't light the burner while you have your hand down there) then you turn the pilot knob to on. It should stay on and you can set the furnace to heat and raise the temperature where you want.
You probably have a defective thermocouple. The thermocouple sits in the flame of the pilot, and the heat generates a small electrical current conducted back to the gas valve, keeping the pilot gas line open after you switch it to the "on" position. That's why you have to manually hold the gas valve depressed (on) during lighting in the "pilot" position, allowing the thermocouple time to heat up enough to generate its current. A defective thermocouple will not generate the holding current.
Move the gas valve knob to the PILOT position and press down on the knob. You will be pressing down on a spring loaded safety magnet. Now light the pilot and keep holding down the knob till the thermocouple gets hot enough to produce electrical current and hold that spring loaded magnet in. This is your safety device. If the heat gets too hot the furnace has a fuse in line with that magnet. When the pilot stays lit by itself without you having to hold down the button then advance the knob to the ON position. Then go to the wall thermostat and turn on the heater.
Pilot lighted furnaces have a thermocouple made from disasimilar metals that when heated create electricity to actuate a relay that alows the gas to flow to the furnace.
When the thermocouple gets coated with carbon from the pilot lights constant burning it acts as an insulator and keeps the thermocouple from getting enough heat to generate electricity, a very good safety factor.
What you have to do is locate the thermocouple it should be located over the pilot light, you will see a gray substance around the thermocouple it will be hard like cement, take a knife and scrape away as much of the gray material as possible then sand the themocouple with a medium sandpaper until it shines, use one of you wifes emery boards, this should solve your problem.
Check regulator on the unit.
Check pilot unit on the heater
use soapy water to check for leaks if see bubbles turn off main gas supply valve.
always turn off gas supply before any checks or repairs are made
when all else failes call your local gas provider to check heater and gas supply
There is a small probe that is heated by the pilot flame. This is called a thermocouple & keeps the gas valve open by generating a small electric current. Most likely the pilot flame is too small, or not heating the thermocouple properly. Either turn the pilot up a bit, or gently bent the thermocouple bracket to put it in the flame a bit more. Do not try to bend the thermocouple itself. If this does not help, a new thermocouple may be needed.