York furnace: system lockout

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Old 03-04-16, 08:53 AM
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York furnace: system lockout

I have a pretty standard setup, a York TG8S gas furnace with a 50M56-843 board. The furnace is maybe 3 years old.
Everything works fine most of the time except when the set point is raised while the furnace is not running. About 50% of the time it works ok but the other times it will enter the system lockout state. As described in the manual the red LED blinks once every couple of seconds (flame sensor fault) until it either resets an hour later or the power is turned off and on again. After a reset everything runs normally until the next time the set point is raised during an off cycle. This has only started happening this winter.
Needless to say it makes it a little hit or miss to run a schedule so that the heat comes on in the morning.
I have had an engineer out twice and the second time he was able to see the problem at first hand. His suggestion was to replace the thermostat for a second time (my first solution was to install a Sensi but it did not make any difference). He confirmed that the wiring was good and that the furnace was fine because when he connected the RH and W wires the furnace fired cleanly.
I'm pretty sure that for the last few weeks the furnace was not even trying to start the whole cycle before locking out. Last night though I tried it again and the flame briefly lit three(?) times before it locked out with the same single blinking light.
So I've been trying to figure out why this only happens in that specific situation and not during a normal fixed temperature cycle. The only thing I can think of is that we might be raising the set point while the furnace is still warm, whereas during a normal cycle it has time to cool down.
Does that sound like a plausible reason for this behavior? If so what specifically could be at fault?
I'd like to have a pretty good theory before paying another 200 bucks for an engineer to come out.
 
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Old 03-04-16, 09:12 AM
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Why don't you test your theory by allowing the furnace to cool off completely before raising the temperature manually? It seems like it has to be either time or temperature related as the control signal from the thermostat is the same whether it is just holding setpoint or you have raised the setpoint. The fact that you saw it try to light three times does seem to indicate the board thinks there is a flame sensor problem. Have you or the tech tried cleaning the flame sensor rod?
 
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Old 03-04-16, 09:36 AM
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The single flash signifies the control board is in lockout mode but it does not tell you why. You need to put the board into diagnostic mode where it displays the last five fault codes before it locked out.

Rather than type it all out..... download the manual for the W/R control in the following link.
The diagnostics access is described on page seven.
emerson climate/documents/white-rodgers/instruction_sheets.pdf
 
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Old 03-04-16, 01:06 PM
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<<. Last night though I tried it again and the flame briefly lit three(?) times before it locked out with the same single blinking light.>>


You have an intermittent problem with the ignition control module not recognizing that the main burners are lit.

When you observed this, did all the main burners light? If so, how long were they on before they shut off?

One simple thing that could be causing this is a dirty flame sensor. The flame sensor is a rod on the side of the furnace opposite the hot surface ignitor with a single wire going back to the ignition module.

Be sure that the flame sensor is engulfed by the main burner flames when they are on. Remove the flame sensor and clean off the invisible coating of oxides that can accumulate on the rod with a wire brush or fine sandpaper. You just need to clean off the surface of the rod so that the flames touch it ---you don't need to cut into the metal.
 
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Old 03-04-16, 02:23 PM
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Thanks for your suggestions. I'll try them this weekend and report back.
 
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Old 03-05-16, 02:55 PM
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Update:
Cleaning the flame sensor seems to have done the trick. Fingers crossed that's the end of this thread!

I still don't understand why the problem tended to happen when the furnace was still warm - never during a constant temp cycle - and why it disappeared as soon as I reset the board by power cycling.

By the way the stored codes were 1, 7, 1, 7 (the order they're displayed). The manual says that's external lockout and low flame sensor current. According to the sticker on the furnace, which I think is wrong, 1=flame present with gas off and 7=ignition lockout due to retries.

Thanks again for the helpful responses!
 
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Old 03-05-16, 06:04 PM
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Low flame sensor level makes a lot of sense and if it was too low on three successive ignition attempts..... the board would go into soft lockout.
 
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