How do I tell if its the thermostat or thermocouple which is bad?

Your pilot would not stay on if the thermocouple was bad. It sounds like an electrical problem. No voltage to the valve is telling you that. Check for any safety's in the system that may have tripped and have to be reset.

Sounds like the thermocouple is bad. The thermocouple is a sensor that is heated by the pilot light. The thermocouple let's the gas valve know that there is a flame to ignite the burner when you turn up the thermostat. When you first light your pilot light by holding down the pilot light button and the pilot light goes out after holding the button for about a minute then the thermocouple is usually bad. It is an inexpensive part normally found it any Box Store.

Thermostat flame sensor. Do you see a little rod next to the pilot? That is what tells the main gas valve that the pilot is lit and it is safe to give gas. Easy swap, and you can usually just pick up a generic one.

If pilot light has gas, then ECO is not tripped so water heater has not experienced overheating event.
Remaining suspects include bad thermocouple, bad air quality, not enough air, bad vent, water dripping down vent, backdraft down vent, hot attic with bad ventilation, low gas pressure, air in gas line, dirt water or debris clogging gas control thermostat, bad gas control thermostat, tripped FVIR, dirty pilot orifice, dirty combustion parts, damaged combustion parts from exposure to trace chemicals like bleach and wax.
Suspects include one or more from list above.

Start with thermocouple.
Open following links:
http://waterheatertimer.org/How-to-test-water-heater-thermocouple.html
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

1) If water heater has electronic gas control thermostat, then the gas control needs to be tested and possibly replaced. Add a comment and describe electronic gas control.

2) If water heater has non-electronic gas control thermostats, problem sounds like the thermocouple. Problem could also be bad gas control.
Make sure end of thermocouple sits in the pilot flame.
http://www.youtube.com/watch?v=v0aBPsNg6vo

Buy thermocouple at local hardware. They vary by length, for example 18".
Thermocouple screws into bottom of gas control thermostat.
Finger tight plus 1/4 turn. No pipe joint compound or teflon tape on threads.
http://waterheatertimer.org/images/IMG_9146-gas-control-thermo.jpg

Heat from pilot flame travels to gas control thermostat.
Heat is changed into small electric current using a thermistor.
The small current tells gas control that pilot flame is ON, and so gas control keeps gas flowing.
If thermocouple is not working, or has fallen away from pilot flame, the gas control turns off gas to pilot.
This safety device is necessary so gas does not fill up house and cause explosion.

http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean

Hello
Sounds like the thermocouple is going bad.The thermocouple is a thin tube that runs from the pilot lite to the gas valve.
What it does is it sences the heat from the pilot lite and keeps the gas valve open to allow the pilot to stay on.
This can be easily repaird by shutting off power and gas to your heater and removeing the thermocouple from the gas valve,then unclip or unscrew it from the pilot lite area.
Any plumbing supply store or home depot or lowes should have these.

Once you get a new one,install to gas valve and pilot lite area then turn on power and gas to heater and follow pilot lighting procedure,you may need to hold gas on for 60 seconds.

Here is a picture of the thermocouple so you know what to look for.


http://thermocatalytic.com/Thermocouple.JPG
http://homerepairgeek.com/wp-content/uploads/2010/12/furnace-thermocouple.jpg

When the whole unit shuts down including the pilot light the thermocouple is not working properly. The thermocouple is what tells the gas safety valve that the pilot light is lighted. When the thermocouple fails and does not send the milli-volt (mv) signal to the gas safety valve due a failure the gas safety valve turns off the gas supply. (i.e total shut down)
If you have a handyman around you can have him disconnect the thermocouple from the gas valve control and clean the mating area in the gas valve and on the bitter end of the thermocouple. After cleaning then reconnect the thermocouple. This sometimes corrects the problem. FYI you can get a thermocouple at most any large appliance parts supply. (You will need to the know total length) It would be even better to have the old one in hand during the trip to the parts store.

Thanks for using FixYa,

Kelly

These thermocouples rarely go bad. The oxygen sensing pilot will get dust in it and shut down making you think you have a bad thermocouple.
Read this page before blowing any money


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Hi

If it won’t fire then it is possible that the oven control is not sending gas for the pilot light. The pilot light works but no main burner ignition - possible pilot assembly is dirty and the pilot flame is too small, safety valve and thermocouple is faulty, the bulb from the safety valve is out of position and the pilot flame is not touching the thermocouple bulb. Some ranges use a standing pilot light ( small flame is on all the time ) while others use an spark ignition to light the pilot light flame and the pilot light flame heats up the thermocouple bulb to allow the main gas to flow through the oven burner.
The flame needs to heat the bulb up enough to tell it to open the gas valve. Several things can go wrong here that keep this from happening:
The pilot flame may not be hot enough, usually because the flame is yellow instead of pure blue or is too small. The cause for this is usually a dirty pilot assembly. The pilot assembly would either need to be cleaned or replaced.
The thermocouple bulb may not be positioned properly in the flame. You can't heat the bulb properly if it's not in the pilot flame! The thermocouple bulb needs to be in the upper third of a pure blue pilot flame--that's the hottest part of the flame.
The thermocouple itself may be burned out. It happens. It's an internal part of the gas valve so, no, you can't just change the thermocouple bulb separate from the gas valve.
But when you turn on the oven or the thermostat calls for heat, the pilot flame gets bigger and jumps down so it can heat up the thermocouple bulb. This extra gas to increase the pilot flame size comes from the thermostat.
If the pilot flame jumps upwards or just gets bigger, but doesn't shoot down, then you need to replace the pilot assembly.
If the pilot flame size does not increase or jump down when turning on the oven thermostat, then the problem is the thermostat not sending enough gas to the pilot assembly. It's also possible that the pilot gas supply tube has a hole in it somewhere.
One final point on the spark-assisted pilot ignition systems. The spark comes from the spark module--the same module that sends spark to your surface burners to light them up. If you're not getting a spark when you turn the oven on, then there are several possibilities:
There could be a problem with the switch in the thermostat. You can confirm this by doing a simple continuity test of the thermostat contacts. If you don't read zero ohms when you turn the switch on, replace the thermostat.
The spark module could be bad. You'll need to measure the voltage at the oven terminals of the spark module when you turn on the oven. If you get 120v but no spark, it's probably a bad spark module. Replace it.
Could be a bad spark wire or broken electrode.

Please do rate the solution and revert for further assistance.


Thanks
Rylee

you probably have a bad thermostat if it wont shut down id replace the thermostat first and then have the thermocoupler checked...replace the thermostat