Carrier 58MXA furnace with pressure switch error


Old 10-28-09, 08:23 PM
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Carrier 58MXA furnace with pressure switch error

OK, so here is my problem. My furnace failed to ignite about 4 or five times during the last heating season. I would notice the house was cold, go down to the basement and find the furnace flashing error code '31'. I would cycle power so that it would call for heat, I would hear and exaust fan come on, but still got no ignition and a '31' error code. I then discovered that if I tapped on the pressure switch a few times while the exhuast fan was running and then wait a few seconds, I would hear the pressure switch 'click' and the burners would ignite and heat the house up to temperature. It would then work for several days until the probelm would repeat, and I would repeat my tapping technique.

This got me through last winter, but the problem seems to be repeating this winter as well (happened twice in a month thus far), so I figure I better get it fixed.

My question is, does tapping technique indicate that the problem is probably a defective pressure switch, or am I mis-leading myself. My theory is that the diaphram is getting worn and the tapping is freeing it up enough to allow it to push closed. I have checked for obstructions and have not found any. When the furnace is running, the intake and outtake PVC ports on the side of the house feel like they are sucking and pushing good airflow, but I can't actually measure it.

I'm just trying to prevent spending $50 on the part only to find out it was something else.

Any recommendations would be appreciated.

The Hoff

Last edited by hoff_b; 10-28-09 at 08:48 PM.
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Old 10-28-09, 09:11 PM
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Is the drain clear?

Old 10-28-09, 10:58 PM
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Nice theory, but only about 5% of the time is a bad pressure switch the problem.

Unfortunately, there are many possible causes of a pressure switch that is failing to close, usually because of a defect or problem in the venting system or the furnace itself.

The only reliable way to diagnose the problem is to use a manometer sensitive to measure down to about .25 inches water column, and someone knowledgeable enough to connect up the manometer properly and interpret the results correctly.

Unfortunately, quite a few furnace repairman don't have the equipment and training to diagnose the problem correctly. If they do not, it often becomes a matter of several trips out as repairmen guess possible causes, and are often repeatedly disappointed by making the wrong guess.

My suggestion: shop around for a skilled repairman who has and uses a manometer to measure the pressure at the sampling port for the pressure switch.
Old 10-29-09, 07:14 PM
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Thanks for the advice. But do you have a possible for reason for why a few taps on the switch will allow it to close? Are there places that can test the switch if I take it in to them?

I have also noticed that the problem usually occurs when the weather turns rainy and windy.
Old 10-29-09, 10:20 PM
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When things are on the edge of failing, any little thing might get it to work or quit at a particular time.

I remember a guy with a gas fireplace that had glass doors that could be opened. He found that if he "flapped" the doors open and closed fast enough, the fireplace would light.

The problem was a dirty pilot burner that didn't heat up the pilot generator enough to generate enough electricity to turn the gas valve on reliably.

So it may be that the pressure switch is on the edge of working. That doesn't explain why. And more likely than not, the problem will get worse over a period of time and it wont turn on at all as it gets a bit worse than it is now.
Old 10-30-09, 07:55 AM
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Why does it happen? Rainy weather?

When it rains the pressure is low and wind often comes from another direction than when you have a high pressure system and sunny weather. Maybe windy air is blowing down your exhaust pipe, in rainy weather, where otherwise it wouldn't be, and that extra little backpressure is juuuust enough to stop the pressure switch from fully closing. Then by you tapping on it, you cause the plunger to jump. And that might be just enough that the contacts are made. And once they are made, they sort of stick together like glue, or a magnet, while the current passes though.

Or, you do not have enough trials of it when rainy or not, and your test data is a statistical anomoly.
(Like flipping a coin only 10 times, and it comes up heads 8 times, rather than 5)

But after rereading your first post, you say after a few seconds, after you have tapped, you hear a click from the pressure switch? Now that strikes me as odd. You'd think it be instant. Are you sure the click is not coming from an ignition control module sending power to the ignitor?

And if the click IS coming from the pressure switch, a few seconds after tapping......WELL then......maybe there is some sort of hang up, maybe with the plunger.

It is possible it could be the pressure switch. But I gave reason why your tapping on it may not conclusively prove that it is.

And if you bought say a different pressure switch that was not the exact one, made by the same company, and if the water column rating was say a tad different..... even an could still actually have an underlying problem, yet thought you 'fixed it' with the pressure switch, when in essence you 'fixed it' similarly to bypassing it.

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