Can't find reason for furnace pressure switch error and lockout

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XUBen

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First post, but have tried everything else I could find with no solution, so here goes.

We have a Ducane 95G1UH/DF Propane Furnace. Installed in 2015 and no real issues since then.

Recently, the furnace works fine, but the indicator light gives three flashes. After 12 minutes or so, the furnace shuts down and gives 6 flashes. Power cycling restarts the process. It'll behave this way for a few days, and then work fine for a while (sometimes weeks).

3 flashes: Pressure switch open with inducer on
6 flashes: Pressure switch cycle lockout

If I bypass the pressure switch (after the inducer starts), the furnace runs fine and no indicator lights.

I tested the pressure switch and confirmed it switches from open to closed after inducer turns on (multimeter goes from OL to continuity, Ohms go from OL to 1.2 and voltage goes from 0 volts to 26 volts).

I pulled all tubes from switch and confirmed they were in good shape with no cracks, had no obstructions and made good seals when replaced.

I checked the condensate trap on the flue and pulled that tube. It did have a tablespoon of water that drained out and some gunk collected around the filter. I cleaned it up and confirmed the tube and filter were clear.

I check all the condensate lines and confirmed they were clear and draining properly.

I checked the flue from the roof. Seems to be venting well and was clear all the way down to the turn in the PVC.

I did notice that the inducer motor seems to get rather hot, but seems to be running fine.

So I'm not sure what else to check and looking for some guidance. What did I overlook or miss? I'm obviously not a professional, but currently unemployed so rich on time (if not a little poor on cash haha) hence the DIY attempt. I appreciate any suggestions!
 

Furno24

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Hello , I have a 2015 Lennox that’s pretty much doing the same thing. Did you ever find out what the root cause of your problem was?
 

XUBen

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Hello , I have a 2015 Lennox that’s pretty much doing the same thing. Did you ever find out what the root cause of your problem was?
Sorry, no. Never did. It seems to have cleared up and run normally since a little after I posted. I suspect it’ll become an issue again… but we’ve been in the clear for a few months now.
 

Fitter30

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Hello , I have a 2015 Lennox that’s pretty much doing the same thing. Did you ever find out what the root cause of your problem was?
What does the burner control flashing light say what pressure switch failed is? Remove tubing and inspect them for cracks and moisture. Lennox doesn't publish for the public much information. Model of furnace?
 
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Furno24

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It’s an ML93UH series from 2015. I tried almost everything recommended so far At night I dial down the temp to 63. In the morning I turn it up to 67. It heats up for 8-11 minutes and shuts off with a ‘Inducer Energized with Pressure Switch Open’ error (3 flashes). This first cycle usually only brings the temp up a degree or two but not to the set point. It restarts and runs a few more times for a short time each before giving the ‘Pressure switch cycle lockout’ error (6 flashes). I recently installed a new inducer because the old one was getting hot and making a loud noise. All the hoses are in good shape and clear of debris. The exhaust is clear. No water buildup anywhere. The pressure switch operates, not stuck or ruptured. I cleaned the flame detector. Next steps are to clean the blower fan and I’m also thinking of putting the old inducer back in to see if it does the same thing. I was thinking it might be the temperature limit switch but if that’s the case I don’t know why I wouldn’t get the ‘Primary limit switch open’ error (4 flashes). Any other suggestions would be greatly appreciated.
 

Furno24

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Sorry, no. Never did. It seems to have cleared up and run normally since a little after I posted. I suspect it’ll become an issue again… but we’ve been in the clear for a few months now.
Thanks, hopefully mine will do the same thing.
 

Fitter30

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With a ohm meter x1 scale touch leads together just to get a base line reading probably .1 . Measure resistance across the closed contacts you sucking on a hose if its .3 or greater replace the switch lennox had problems with swiches made in Porto Rico in age of furnace.
 

Furno24

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Thanks. I have a new pressure switch coming so I’ll see if that does the trick.
 

MaverickInventor

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First post, but have tried everything else I could find with no solution, so here goes.

We have a Ducane 95G1UH/DF Propane Furnace. Installed in 2015 and no real issues since then.

Recently, the furnace works fine, but the indicator light gives three flashes. After 12 minutes or so, the furnace shuts down and gives 6 flashes. Power cycling restarts the process. It'll behave this way for a few days, and then work fine for a while (sometimes weeks).

3 flashes: Pressure switch open with inducer on
6 flashes: Pressure switch cycle lockout

If I bypass the pressure switch (after the inducer starts), the furnace runs fine and no indicator lights.

I tested the pressure switch and confirmed it switches from open to closed after inducer turns on (multimeter goes from OL to continuity, Ohms go from OL to 1.2 and voltage goes from 0 volts to 26 volts).

I pulled all tubes from switch and confirmed they were in good shape with no cracks, had no obstructions and made good seals when replaced.

I checked the condensate trap on the flue and pulled that tube. It did have a tablespoon of water that drained out and some gunk collected around the filter. I cleaned it up and confirmed the tube and filter were clear.

I check all the condensate lines and confirmed they were clear and draining properly.

I checked the flue from the roof. Seems to be venting well and was clear all the way down to the turn in the PVC.

I did notice that the inducer motor seems to get rather hot, but seems to be running fine.

So I'm not sure what else to check and looking for some guidance. What did I overlook or miss? I'm obviously not a professional, but currently unemployed so rich on time (if not a little poor on cash haha) hence the DIY attempt. I appreciate any suggestions!
Bypassing the switch eliminated the problem. The switch is the problem, or negative pressure was not present each time it flashed 3X. Easy to zero-in on a solution from here.
 
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