Originally Posted by Myrle
When you hold down the pilot/burner knob you are heating the thermocoupler, what happens is a thin wire expands and opens the gas valve at the burner. ...
Nope. The thermocouple, when heated, generates a small (about 25 millivolts) DIRECT CURRENT voltage. This voltage (think small battery) drives a circuit running through a pilot solenoid coil in/on the gas valve. The current running through this pilot coil produces a magnetic field - that magnetic field holds open a small valve that provides gas to the pilot burner.
If the pilot light, for any reason, goes out - the thermocouple cools, the current drops, the pilot valve closes, and gas stops flowing to the pilot burner.
The reason you have to hold the button down when lighting the pilot, is to mechanically open the pilot gas valve. You then have to keep pushing down until the thermocouple has a chance to reach a high enough temperature, to develope enough current flow, to allow the pilot solenoid's magnetic field to do the holding.
Generally, the problems with standing pilots are:
1) No gas to valve. Disconnect the pilot gas tube at the gas valve and depress the button. If gas is present at the gas valve and not at the pilot, check the pilot tube and pilot assembly orifice for blockages; clean or replace as necessary
2) a blocked pilot orifice. The pilot orifice is at the end of the small gas line going to the pilot. The come in different configurations, but all have a very small hole through which gas is metered at the pilot burner. For some reason, it seems small spiders love to spin webs here. Although reaming is not recommended due to the possiblity of damage to or changing the size of the orifice, I have had great luck using small, thin plastic (like a bristle from a fine hair brush), to reopen this orifice.
3) corroded or misaligned thermocouple (polish the surface a bit with some very fine sand cloth, and with the pilot flame ignited, about 3/8 to 1/2 inch of the thermocouple tip is actually enveloped by the pilot flame. You can make mechanical adjustments to its positioning by moving the thermocouple back and forth in its mount or side to side by slightly bending it bracket.
4) corroded or oxidized connection of the thermocouple line connection at the gas valve. You can unscrew the "tube" going from the thermocouple to the valve (it's an electric connection - not gas). You will see a small "tab or hat" at the end of the inner part of this tube. if you look closely, you will see an insulating washer that keeps it from contacting the outer part of the thermocouple tubing. Polish this tab with the with same sand cloth you used for the thermocouple.
By the way - you can check to see if the thermocouple is generating enough voltage by manually lighting the pilot (remember - hold the gas button down!), let the thermocouple get hot, then measure VOLTAGE between the tab and the outer part of the thermocouple tube. If there's at least 20 to 30 mV DC present, the thermocouple is operating satisfactorily.
4) a bad pilot valve solenoid. The button end of the thermocouple is screwed into the top of a device that looks like a small can with a plunger coming out its end. There is a gasket on the end of this plunger, and a spring that keeps the plunger extended. You can remove this component from the main valve by unscrewing it. (remove the thermocouple line first - and turn the gas off because this device DOES provide a gas seal). If you passed the check in (4) above, you can test this "can". It involves removing the thermocouple from the pilot burner, and the solenoid from the main valve, screwing the thermocouple tubing back into the solenoid, taking the assembly to your gas stove and holding the thermocouple in the stove burner. Once the thermocouple is hot, you should see the plunger has pulled in. If it did not, the solenoid can be replaced as a separate part. (a trip to a small appliance parts shop).
There are, of course other things that can go bad, but probably 90% of the time those above are the standing pilot faults.