Pilot wouldnt stay lit. replaced the thermocouple and now pilot wont light at all.

The system I am familiar with uses a vapour pressure type thermocouple and if the pilot remains lit the thermocouple is working as it controls the gas for the pilot and the burner at the same time from the same source - if the gas control is ok and the flow isn't prevented by the thermostat then if the pilot remains alight, there should be a flow of gas and the burner should light - unless the gas jet is blocked or something else, some other type of blockage is preventing the flow of gas to the burner...

I suggest you check the gas jet and the flow of gas to it...

if the therocouple has packed up pilot light will not light thermocouple has long small diameter pipe leading to a larger end this needs to be above the pilot flame also for safety have a small bowl of soapy water with a brush wet the nut with soapy water in no bubbles appear then try lighting heater thermocouples don't last that long

I would remove pilot light assembly and clean orifice and blow out gas line to pilot. Check to see that thermocouple is centered in pilot flame. It is possiblethat your gas valve is bad but it is also possible that you got a defective thermocouple. I would change the thermocouple again as they are much less expensive then the gas valves. The next time when you are sure you have the thernocouple mounted as close to the center of the pilot flame hold the pilot button down for several mins. If none of this works then you may have to change your gas valve.Thank you.

You need a new thermocouple - or maybe the Pilot flame is not lined up properly to the thermocouple? The thermocouple is a "sensor" that is heated by the flame - which in turn keeps the gas supply to the pilot light on. It the pilot light goes out, then the thermocouple no longer "senses" the heat of the flame, so it turns the gas supply off to the pilot to prevent buildup and explosion.

Hi,
First thing is to replace the thermoscouple. When you light it you get it a bit hotter then normal. You turn on the main burner which keeps it hot. Then when it goes back to just pilot the flame is not hot enough to hold the pilot valve in.
Replace the thermocouple and you will be fine...

heatman101

Possible thermocouple is defective. The thermocouple is heated by the pilot itself, the theory behind this safety design is so the main burner cannot come on, if the thermocouple won't stay hot, it reduces the chance for a huge gas buildup in the burner compartment and possible explosion.

I have also seen cases a pilot would not stay lit if the flue for the heater is blocked by a spider web, bird nest or other obstruction.

Check and make sure the flue/vent is absolutely clean and clear, if it is, then the thermocouple may be defective. If you have an electric pigtail on the thermocouple, the problem may be the electronics or circuit board.

Here is a link to a photo of a thermocouple
https://www.centraltrailer.com/cart/images/2631-3481.jpg

replace the thermo

get a long stick match, a flashlight remove lower inspection cover, find thermocouple and pilot light will be next to it,turn gas valve to pilot light and hold down,stick the match in by thermocouple till the pilot lights and hold valve down till pilot stays lit,this may take a minute or more,then release gas valve and see if pilot stays lit, if not retry,if it wont stay lit,the thermocouple is defective and needs replacement

The pilot assembly should look similar to the pic above. Depending on where you live, you may be required to hire a certified plumber to replace the parts. If not you can do it your self. Keep in mind if it's not staying lit you also need to check the thermostat and the thermocoupler before replacing the pilot assembly. There is a good chance it is just the thermocoupler going out. It is one of the first things to check when troubleshooting a pilot light problem with a gas water heater. A thermocouple determines if the pilot light is lit. If it's not lit, the gas valve fails. If that's the problem, replace the thermocouple.

1. Shut off the gas valve on the water heater and the gas valve on the line near the heater. Remove outer and inner doors, and loosen the pilot and supply tubes and the thermocouple connections to remove the burner assembly.
2. Step 2 Loosen the screw holding the thermocouple to the bracket. Remove old thermocouple. Bring it with you to the store to ensure you are buying the proper replacement.
   
3. Step 3 Install the new thermocouple. Position the thermocouple so that the pilot flame heats 1/2 inch of the tip.
   
4. Step 4 Reinstall burner assembly in the water heater. Tighten burner supply tube, pilot tube and thermocouple connections. Tighten thermocouple no more than a quarter turn more than hand-tight.
   
5. Step 5 Turn on both gas valves. Check supply tube and pilot tube for leaks with soapy water. If soap bubbles, tighten connections to fix leak. Ignite the pilot light, and make sure the thermocouple tip is heating up. Replace inner and outer doors.
   

The thermocouple or gas valve needs to be replaced. Start with the thermocouple.

Good Luck