The ignitor clicks but no gas to heat thermocouple

You have to hold the button down in the pilot position to allow gas to go to the pilot while you click the igniter. Hold the button down for a minute or two to allow the thermocouple to heat up, then turn the knob on the gas valve to the normal position. It should work providing that the pilot is capable of remaining lit. If not, then it could have a defective thermocouple or gas valve (aquastat).

If pilot stay lit, then thermocouple is working.
If pilot will light, then you have gas. If pilot goes out when pilot button is released, then thermocouple is next suspect.

Thermocouple tip must sit in pilot flame.
The pilot light heat applied to two different metals inside thermocouple causes small current.
The current travels back to gas control valve through copper line.
If the gas valve receive this current, then a magnetic switch stays open and allows gas into the gas control valve.
The magnetic switch is on the end of the red button you push down when lighting pilot. Essentially the red button is an override to the magnetic switch.
If the gas control valve does not receive the current, then the magnetic switch does not stay open when you release the red button.
When replacing thermocouple, note that it is not screwed very tight into the gas control valve. Finger tight + 1/4 turn. If it is overtightened, then the small thermocouple current shorts out against the gas control body.

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1) Open following link for how-to resources, illustrations, and service manuals for some brands of heater.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

2) Many things can cause pilot problems, including bad thermocouple, air in gas line, low gas pressure, negative pressure in house, bad venting, very hot attic, lack of combustion air, dirty combustion parts and dirty pilot tube, bad gas control, reversed polarity on electrical outlet.

3) If pilot flame is clear blue and strong, then next step is check that thermocouple sits in the flame, and then change thermocouple. This is start point. Following step would be reading service manual for exact troubleshoot and testing steps.

4) If pilot flame is yellow-or-orange or weak, then clean dirty combustion parts including pilot tube and air intake. Might want to change thermocouple while combustion parts are removed for cleaning.

5) Take a moment and rate answer:
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sounds like the thermocouple:
The thermocouple is a safety device that turns off the gas if there is no pilot light. If you look closely at the burning pilot light, there is a small metal tube that sits directly in the flame, with wires that lead back to the gas valve. The thermocouple generates a small amount of electrical current when heated by the flame. This current keeps the gas safety valve open. If the pilot light gets blown out, the current stops and the gas valve closes. When you light the pilot light, you hold down the over-ride button for about 30 seconds. This allows the gas to flow long enough to heat up the thermocouple and generate current and then you can release the over-ride button and gas will continue to flow. If the thermocouple is defective, then as soon as you release the over-ride button, since there is no electrical current, the gas will stop flowing and the pilot light will go out.

This is a pretty common failure on water heaters and furnaces. Replacement of the thermocouple is fairly simple if you are the handyman type. If not, the thermocouple is a cheap part -- about $10 bucks, but the service call may run you $100 or so. Before you call a serviceman, get all of the brand and model information from the information plate on the heater so he can bring the proper replacement part. And sorry for your trouble. hope this helps:)

When you removed the burner and replaced it did you bump the thermocoupler? If that isn't in the pilot light flame or ignitor heat, it will not allow your gas valve to open and send gas to your burners.

If it isn't misaligned then it could be bad or just corroded/caked and need cleaned.

Hi, I work in a plumbing store and that problem you described is related to a piece called thermocoupling, its a coppery thin line that heats up and controls the flow of the gas. If not this then I would check to see if your flue pipe is a bit clogged as the heat is not able to flow up quickly enough. Another culprit can be a problem with the vent pipe on the roof, as a non properly functioning vent will let draft down to the flue pipe. Most likely I think it is the thermocoupling though. Hope this helps!

Some things to check:

Make sure the thermocouple is squarely in the pilot flame and that the pilot flame is large enough to envelop the thermocouple. A weak pilot can be caused by gas pressure or a carboned-up pilot orifice (where the gas comes out). The orifice may need to be cleaned. Ordinary automotive carburetor cleaner will work. Don't try to stick objects into the orifice - damage may result. If you need to perform this step, make sure the gas is turned off to the heater first.

Make sure the other end of the thermocouple is clean and tight in the gas valve.

Hold the pilot plunger down for at least a minute to be sure to give the thermocouple enough time to heat up.

As long as you have a good strong pilot flame under thermocouple in the first place, try this. Shut gas valve off, and clean thermocouple above pilot flame. Best way is to remove it, and clean it with fine steel wool, or fine emery cloth, at both the probe end that sits over pilot flame, as well as undo it from gas valve and rub contact surface on other end with steel wool as well, and if you have any electrical cleaner, give it a squirt in spot where thermocouple was removed from, and clean it out well, and if no electrical cleaner, roll a piece of steel wool up small enough to get it in that hole and twist it on the surface where thermocouple meets, to clean the surface . If that works, great, if not, either thermocouple is defective, or bad gas valve.

This sounds like a dirty pilot jet orifice or a weak thermocouple. The pilot orifice can carbon up (natural gas is a fossil fuel), become weak and keep going out whenever the main burner lights, which causes a pressure drop in the gas line. The thermocouple can also become dirty and not sense properly.

The pilot light assembly should be cleaned. The gas must be turned off and it's best if the pilot assembly can be removed. Automotive carb cleaner can be used to clean the orifice. Do not try to insert any object into the pilot orifice valve as this can result in damage.

If cleaning does not resolve the problem, replace the thermocouple first. Make sure it is installed correctly and is squarely in the pilot flame.

Once the flame has struck it heats up a thermocouple that in turn holds the gas valve open

It is a safety measure in case the flame blows out and the gas keeps on coming.

Sometimes the end of the thermocouple gets dirty and a scrub with a fine emery board does the trick

If not it is a new part (They are not that expensive)