This is an older Bryant furnace, model 394GAW.

It has electronic ignition. Pilot lights fine and stays burning when thermostat
Requesting heat. Burners ignite, but then only for about 4-5 secs, then go out. This
Will repeat about every 30 secs or so. The blower never starts.

I found numerous posts on internet about replacing the 3 wire pilot/flame sensor, which I did, and it didn't make any difference.

I checked the circuit to the gas valve, and on the leads by the valve to the burners,
It will read 0v and then 24v and the burners will ignite (assuming that this opens
The gas valve to the burners) returns to 0v in about 1 sec and burners then go out.

Any suggestions?

To me it sounds like you have a weak main limit switch. I assume the flame goes out before the blower comes on. The 5 wire gas valve uses a pick and hold circuit and will show this symptom when it goes bad.

T-Top,

Yes, the burners go out before the blower comes on. The pilot remains on though.

Is there a way to test or jumper the main limit switch? Is that the rectangular
Shaped panel with two terminals on it in the heat exchanger box?

Thx

Quote:

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Originally Posted by tbez357

T-Top,

Yes, the burners go out before the blower comes on. The pilot remains on though.

Is there a way to test or jumper the main limit switch? Is that the rectangular
shaped panel with two terminals on it in the heat exchanger box?

Thx

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Yes that is your main limit. You can use a jumper wire to bypass it for testing only.

Quote:

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Originally Posted by T-Top

Yes that is your main limit. You can use a jumper wire to bypass it for testing only.

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Bypassing mail limit didn't make a difference. Also, if main limit is bad, wouldn't pilot go out too when main trips? (it doesn't)

If the gas valve is chattering when the main burners try to come on you may have a bad gas valve. The 5 wire gas valve with a 3 wire pilot burner is a hard system to trouble shoot even for the moste seasoned technicians..

Quote:

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Originally Posted by T-Top

If the gas valve is chattering when the main burners try to come on you may have a bad gas valve. The 5 wire gas valve with a 3 wire pilot burner is a hard system to trouble shoot even for the moste seasoned technicians..

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T-Top,

I figured out the problem. I noticed the gas hot water tank burner didn't sound quite right when it kicked on, which made me think in a different direction. I called the gas company. The main gas flow/pressure was down. The valve at the meter was partially closed (I think it may have been stepped on by contractors when they replaced the shingles on roof), and the gas company blew-out the line. Furnace works like a charm now. Unfortunately I replaced the pilot unnecessarily, but it's working which is all that matters.

Quote:

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Originally Posted by tbez357

T-Top,

I figured out the problem. I noticed the gas hot water tank burner didn't sound quite right when it kicked on, which made me think in a different direction. I called the gas company. The main gas flow/pressure was down. The valve at the meter was partially closed (I think it may have been stepped on by contractors when they replaced the shingles on roof), and the gas company blew-out the line. Furnace works like a charm now. Unfortunately I replaced the pilot unnecessarily, but it's working which is all that matters.

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Glad you have heat now. That's the first thing a tech will check is in put and out put of gas pressure at the gas valve. Stay warm.

Quote:

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Originally Posted by T-Top

Glad you have heat now. Thats the first thing a tech will check is in put and out put of gas pressure at the gas valve. Stay warm.

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The pilot burned strong, so I never gave it a thought. Interesting that I must have
Found close to a dozen posts on problems similar to mine, and not one mention of
Checking the gas pressure. I guess this could be the first one, and hopefully it will help somebody else. I won't forget it.