Gas Furnace Turns Off After Pilot Light

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Old 03-13-20, 08:16 PM
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Gas Furnace Turns Off After Pilot Light

So I am at my wits end here. We have a gas furnace that for some reason is kicking off whenever the pilot light touches the flame sensor. I thought it might be the gas valve not glowing gas to the burners but it seems like the furnace is not even getting to the point to get the gas to the burners. If you disconnect the wire from the flame sensor the pilot light start and continues burning. But the moment the flame touches the flame sensor the whole thing kicks off. I have checked all the wires with a multi-meter and there are no breaks in the wires. I also checked the ground wire and it doesn't seem to be like there's any breaks and that either. I even tried cleaning the sensor with super fine steel wool and even 160 grit sandpaper. Does this seem like it's a faulty flame sensor? I've included two links below true some videos I took of the situation. The first one shows how the pilot light won't burn until I put the flame sensor into it, and the second one shows how the whole system will turn if the pilot light flickers and touches the flame sensor.

Pilot light, then flame

Normal Cycle, Then power Off
 
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03-14-20, 01:18 PM
roughneck77
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Check your heat exchanger first. If the heat exchanger is ok you can proceed further with troubleshooting.
You can use your meter to test the flame rod.
Either way with a 25 year old furnace you should start planning ahead of time for its replacement.
 
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Old 03-13-20, 08:30 PM
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What brand and model furnace is this?
 
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Old 03-13-20, 10:58 PM
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In the video, why are you testing the assembly outside of the furnace and the flame sensor in your hand? That's pretty dangerous especially if you have the main gas valve wire connected.

What's likely happening is it's momentarily sensing the flame but the signal dropping below the threshold. Module/control is detecting the flame dropping out and restarting the cycle.

 
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Old 03-14-20, 08:25 AM
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Lennox
M/N: G23Q2/3-75-2
SN: 5895K 00366
 
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Old 03-14-20, 08:32 AM
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Yeah, I agree that it probably wasn't the best idea but I wanted to trouble shoot anyway I could. So would this all be more than likely the result of a faulty flame sensor?
 
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Old 03-14-20, 08:41 AM
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No, not necessarily.
That furnace came from the Marshelltown plant in October of 1995. Has the heat exchanger been checked? If it’s cracked or rotten your furnace is dead no matter what the pilot light does.
 
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Old 03-14-20, 09:00 AM
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Flame sensors don't really go bad - it's a piece of metal.

Just needs to be clean and the ceramic insulator can't be damaged.

Bad connections, ground or dirty pilot assembly can cause issues sensing the flame.
 
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Old 03-14-20, 11:51 AM
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ok so I have cleaned the flame sensor, cleaned the pilot, and tried two new ground wires. But it is still shutting off the pilot and not opening the gs valve on the flame touches the flame sensor. What would be my next thing to test and how do I test it?
 
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Old 03-14-20, 12:11 PM
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See my previous post before going further.
 
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Old 03-14-20, 12:25 PM
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On that model, you can pull the blower assembly and look up to see the heat exchanger.
Looking for cracks and popped rings.

If you don't have an a/c coil right above, you can cut a hole in the plenum and inspect from there.
 
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Old 03-14-20, 12:42 PM
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So at this point should I worry about checking the heat register or should I start planning on getting a new furnace? Luckily I have enough space heaters to make it through the rest of the season quite easily.
 
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Old 03-14-20, 01:18 PM
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Check your heat exchanger first. If the heat exchanger is ok you can proceed further with troubleshooting.
You can use your meter to test the flame rod.
Either way with a 25 year old furnace you should start planning ahead of time for its replacement.
 
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Old 03-14-20, 07:28 PM
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So I no longer think it is the flame sensor. My neighbor had a spare flame sensor. I plugged it in and still had the same results.
 
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Old 03-14-20, 07:30 PM
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Check for a compromised heat exchanger first. If it’s cracked or rotten it’s pointless to do any more troubleshooting to the furnace.
 
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Old 03-16-20, 08:52 PM
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So I check d the heat exchanger and I was not able to find cracks. One of my friends is a handyman and he said that it might be the control board. Is there a way to trouble shot that?
 
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Old 03-17-20, 03:24 AM
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Where did you look for cracks? They are usually mid way up along the back seams in that model.
Use your meter to check the mA signal from the flame rod. Don’t let your neighbor talk you into changing parts with no diagnosis.
 
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Old 03-17-20, 07:51 AM
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I'll check it myself when I get home, but he stated that he checked with sticking his hand up each one and by checking with a light. I checked the flame sensor and it tested all right. I misspoke earlier though. This is the part that he said it malfunctioning. It said that the flame sensor is sending the signal to this control board and it is shorting out before it can send power to the gas valve.

https://www.supplyhouse.com/Honeywel...SABEgLFA_D_BwE
 
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Old 03-17-20, 07:55 AM
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What was the reading you got when checking the flame sensor?
That’s not the correct way to look for a heat exchanger crack. They usually form along the back near the seams. Pull the blower and look up, or if there’s no air conditioning cut a hole in the plenum and look down.
 
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