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mranswer, HVAC Mechanic
Category: HVAC
Satisfied Customers: 6157
Experience:  20 years experience running my own business
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I just replaced 36C03 gas valve on my furnace after a flood

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I just replaced 36C03 gas valve on my furnace after a flood and cleaned out pilot light tube. I get a nice blue pilot light but I can't get it to stay lit after I release pilot knob. I already replaced thermocouple and it didn't help. What else can cause the valve to shut down pilot light?
I will do my best to help you with your issue

Since you've replaced both the thermocouple and the gas valve the only other causes for the pilot to go out when you release the pilot light button are a pilot flame that's too small to heat up the thermocouple. A thermocouple that's not properly positioned so only the top 1/3 is in the flame or a bad thermocouple connection at the gas valve (did you use pipe dope on the threads?)
Customer: replied 3 years ago.

Thermocouple heats up to a red color. Flame covers about 90% of its length I didn't use pipe dope on the threads. I also measured voltage on my old thermocouple and it went up to 32mV. Didn't measuse it on new one, figured it wasn't the problem.

If the flame covers 90% of the length of the thermocouple then that's probably your problem. Thermocouples must maintain a cold junction in order for them to work. This means the bottom 2/3's of the tip should not see any flame.
Customer: replied 3 years ago.

I pulled it out so the flame was covering only 1/3 of the tip and tried going through start up process several times. It still doesn't stay lit.


Is it too much moisture in the air or gas line needs to be purged?


If you have a flame then air is not an issue. If you read 32 mV on the old thermocouple then it wasn't bad. I've listed all the possibilities. You said you didn't use pipe dope on the threads but is the connection at the gas valve tight? If so then we've covered all the bases; new gas valve, new thermo, 1/3 of the tip only in flame, strong pilot, thermo is hot, tight connection at the gas valve. There isn't anything else that I can think of. If you've done all this all I can do at this point is to opt out and see if one of my colleagues has any ideas
One other thing, it's a reach but; is this a typical thermocouple? No terminal block in the thermo? just a straight bare copper wire?
Customer: replied 3 years ago.

It's plain thermocouple labeled:


18" Universal Thermocouple

Opting out. Please do not respond until you get a response from another expert

Please check the millivoltage on the new thermocouple, or put the old one back in. This sounds like the new thermocouple may have either been bad from the start, or is now bad. Rick has covered all the bases. Your new t couple should produce at least 20 millivolts.

Customer: replied 3 years ago.

I assume by "thermopile" you are refering to thermocouple. I disconected it and hooked it up to multimeter. Turned on the pilot and it went up to 28mV and was still slowly climbing up.

Yes, I had edited my post just soon after I proof read it.

Did you replace the gas valve with the exact same model? It sounds as though there is no connection being made at the valve from the couple. Also check to make certain, none of the control wires are accidentally being ground out. This will prevent the pilot from staying on.

Customer: replied 3 years ago.

Just checked TH and TR and they are not connected to the ground.

My old gas valve was 36CO3 Type 221.

The replacement is 36C03-300. I used 1/3" to 1/2" adapter on the outlet line. The box was labeled with adater included, but it wasn't in the box, I had to buy it at Home Depot.

Try removing all wires from the terminals on the control valve and only leave the thermocouple connected, see if this will allow the pilot to stay on. Do not over tighten the thermocouple, this is a simple wire connection and not a gas tight connection.
Customer: replied 3 years ago.

Do you want me to remove E.C.O. jumper as well?

No, you must leave the eco jumper in.
Customer: replied 3 years ago.

Disconected TH and TR wires and made sure thermocoupler connection is not overly tight. Did'nt help.

Ok, I will have to opt out. It sounds as though your new gas control valve is bad, I cannot think of any other tests that you can do, or any other problems that may cause this.
Customer: replied 3 years ago.

Thanks for your help. Hope someone else has the answer.

The pilot holding coil on your new gas valve is bad. You need another valve. There is nothing complex here as you measured the right voltage from the t/cpl so the only reason the valve will not allow pilot flow is because the holding coil is not operating. Rare to get a bad valve, but there is no doubt in my mind that this is what is wrong.
Customer: replied 3 years ago.

I am coming to the same conclusion. I will order another valve by express shipping and if that's the case I will give you satisfied answer. Will have to wait a few days and see

OK- this issue is as simple as a light bulb and a socket- power to the socket means the bulb is bad if it does not lite up. Nothing mysterious here.
mranswer, HVAC Mechanic
Category: HVAC
Satisfied Customers: 6157
Experience: 20 years experience running my own business
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