Carrier error code 31... but everything checks out


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Old 10-18-15, 03:20 PM
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Carrier error code 31... but everything checks out

Carrier gas furnace model 58PAV111-16

I have an error code 31, and I have eliminated every possible cause that I can find suggested online. There are no blockages. I've been through that before. I opened it all up and looked closely... removed tubes which are clear... and I've even eliminated a blocked chimney as a possible cause by trying to start the furnace with the stack removed from the top (a test I do not recommend). The inducer motor is providing suction to the pressure switch and the switch is operating correctly. I checked resistance between the two terminals on the pressure switch with the furnace switched on and the inducer motor running... The switch closed normally. I will try jumping the leads to the pressure switch and starting the furnace, but there is no logical reason why that would change anything.

A systems check showed that everything is working... inducer, igniter, blower, etc.

I am no novice to working on this furnace. I've fixed code 31 before, changed limit switches, replaced the control board once, and I've even had the whole thing apart to replace the baffles. This time I'm stumped.

Can a bad control board cause the error code 31?

Thanks for any advice.
 
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Old 10-18-15, 03:57 PM
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What you appear to have been doing so far is guessing that various things might be causing a problem,

You need a more scientific approach.

For that you need a manometer with a sensitivity of a tenth of an inch water column or so.

Connect that up correctly and you can measure the negative air pressure being applied to the pressure switch. You also need to know the negative pressure which should cause the pressure switch to close --- you should be able to get that from the manufacturer by providing them with the make, model and serial number of the furnace.

Very likely there is a defect in the venting system someplace. There are lots of things that can be wrong.

Just to mention a few you didn't mention, you might have an i9nducer motor not coming up to speed, a fand wheel that is rusted or damaged or the air sampling port the pressure switch hose connects to could be plugged with rust or debris.

Disconnecting both the inlet
 
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Old 10-18-15, 04:23 PM
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I had read about Carrier having a TSB on this line of furnaces. They added a moisture collection "T" to the intake PVC elbow at the furnace entry to keep the moisture out of the draft inducer.

I couldn't find any mention of it at Carrier. It would appear to be an older TSB.

Seattle Pioneer lists the appropriate way to diagnose the system.
With a code 31 showing.... the igniter should not be operating.

The most common cause of problems is the little metal nipple where the rubber hose connects to the inducer gets plugged.
 
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Old 10-18-15, 06:01 PM
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Thanks for the responses...

There is no pvc intake for this furnace. The upper door is vented like all old gas furnaces, and it draws air from the room (basement).

I did check the nipple for blockage, and blew back through the vacuum hose. It was open. I will retest the pressure switch operation tomorrow with the test that should have shown that there is no blockage or weak motor, and that the pressure switch is working. Connecting my meter to the pressure switch terminals with the furnace running the inducer, proved that the switch was closing correctly... which proved that there is enough suction to operate the switch. When I retest it, I'll leave the meter on and let the inducer run longer to make sure there is no break in continuity during a longer run. I suppose it is possible that the diaphragm in the switch could throw the switch initially, and then perhaps a small hole could let the switch open again. That is a long shot though because I did watch the meter the first time for about 30 seconds. This pressure switch requires only a tiny amount of suction to close. I would imagine it is very unlikely that the contacts inside the switch ever corrode inside a sealed area like that, unless the diaphragm does have a small leak.

Error code 31 is listed on the error codes on the door, along with the likely causes. I've not really been guessing... just eliminating each possibility. The last time I had this code, the chimney cap had come off and a bird carcass was inside the furnace. I wish it were that simple this time. I replaced some of the chimney pipe last year when I found a hole rusted through, and I cleaned the whole venting system out then. There was almost no debris this time, except a couple of dead beetles which were not near anything that could be blocked.

I've called repair guys only twice in the last 38 years. lol Once was almost that long ago for a broken refrigerator, and the last time was several years ago for this furnace. A baffle had cracked right next to the limit switch, and I didn't see it. The baffles were under warranty, but labor wasn't, so I changed all of them myself. Hopefully there wont be a third time.
 
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Old 10-18-15, 06:05 PM
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Connect your meter to the pressure switch before the call for heat. There should be 24vac across the pressure switch contacts. Once the inducer starts... that should go to and stay at 0v.

If your control board has one of those 9 or 12 pin plugs..... unplug that a few times. They can become oxidized.
 
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Old 10-18-15, 08:10 PM
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I'll give that a try tomorrow after work... thanks
 
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Old 10-19-15, 04:18 PM
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Okay... I have 4 volts across the pressure switch which drops to a constant .25 volts when the inducer runs.

I should note that I'm starting the cycle by switching on power to the furnace with the fused switch on the side, and when power is switched on the main blower runs for a couple of minutes, stops, and a couple of seconds later the inducer starts (normal operation for this furnace when the power has been off). It probably doesn't matter but I mention it in case the main blower running affects pressure switch voltage. I didn't notice much of a voltage change when the main blower stopped.
The LED shows a code 12 when the blower is running, showing that the furnace powered up from a state of power out with the thermostat calling for heat. If I leave it run, it will cycle from that, to the inducer motor running with a code 31, and back again.

I also tested the hot terminal of the pressure switch to ground... 28 volts. The other terminal showed no voltage to ground, and went to a constant 28 volts when the inducer started. This was expected since I've already proven that the pressure switch is engaging with the earlier resistance test.

I also pulled the pin plug and slid it on and off a few times. The pins are clean and shiny and nothing changed.

Is the extra four volts going to the pressure switch, and the four volts across when it should be zero the problem?

Thanks again for the help.
 
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Old 10-20-15, 01:30 PM
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I'm curious... If I should have seen 24 volts between the two pressure switch terminals with the switch not engaged, and I know that the insulated terminal connector is 24 volts positive, doesn't that mean the other terminal should be connected to ground? If that is the case, then only showing 4 volts suggests a bad ground... right?

I expected that since one side was hot before the pressure switch engaged, and the other side was only hot after the switch engaged to complete the circuit, then the system was likely sending 24 volts to the board to verify that the inducer is running and the ignition process could begin safely. I'm not at home to trace the wiring, and the schematic makes me go cross eyed lol... but if one side is hot and the other connects to ground, wouldn't the pressure switch cause a dead short when it engages? I once saved a $300 service bill by repairing a power supply board in a television by replacing some five cent capacitors, but that required no understanding of how everything electronic works.
 
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Old 10-20-15, 02:21 PM
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To confirm your measurements.... disconnect the wires from the switch and then measure again.
I'm fairly certain you should see near 24vac.
 
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Old 10-20-15, 04:59 PM
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Thanks PJmax...

It was the Draft Safeguard Switch. My brother is pretty good at reading schematics, and he saw the DSS on the same circuit as the pressure switch... just after it. I pushed the reset button and that fixed the problem.

Now I'll have to worry about why that switch activated, but I'm almost certain that it is not a blocked chimney, and it wasn't windy the day this happened. I'll wait to see if it happens again before I look further into that, and I'll be watching that the chimney is venting properly when I need to turn the furnace on again. It was 80 today, and will be warm until this weekend.

You actually helped without knowing it. The question about the pressure switch current led me to the schematic. Next time I'm working on the furnace I will test between the pressure switch leads... and I'll test the non powered side continuity to ground... but I'm pretty sure these safety switches only break the 24 volt circuit.

I can't believe I hadn't found this forum sooner. It is a great help.
 
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Old 10-20-15, 08:29 PM
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I would check the vacuum at the pressure switch if the spill switch is tripping.
I would expect ~ .66" water column negative pressure but you want at least over .3" W.C. of negative pressure with a 58PAV.




 
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Old 10-21-15, 01:45 PM
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Pretty much anything that impairs the flow of ventilation/combustion air through the furnace can cause that switch to trip.

Take a careful look at the switch and observe where it's located. It will trip if hot combustion gasses wind up being forced out in contact with the switch.

Removing and inspecting the inducer motor fan/assembly is worth doing. Usually that's fairly easy to remove. Check the fan blades, motor and the parts of the heat exchanger you can see.


Personally, I favor grounding one side of the AC voltmeter to furnace sheet metal ground. The other probe can then be easily moved from location to location to see if you have zero or 24 VAC at various locations.


Measuring between a switch, where 24 VAC may mean the switch is open and zero volts that it's closed is an invitation to confusion, for me.
 
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Old 12-29-15, 07:40 PM
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Back again... Same code 31

First.. I see there were a couple of responses after I found the bad draft safety switch. Thank you all.

Unfortunately I once again have a code 31 after weeks of the furnace working well. Last time, I replaced the resettable draft safety switch with a correctly rated but single use one., I checked continuity and it has not blown, so the DSS is not the problem this time. Once again, everything seems to be working properly, but now the furnace runs for about five minutes, shuts down well before the thermostat shuts it down, then restarts after about a minute... And that process continues indefinitely, or until the thermostat isn't calling for heat. It's not windy and the vent is clear. The inducer seems to be working normally. Although I have no tools to check the exact pressure to the pressure switch, I did connect my meter to the non-powered side and tapped on the switch, with absolutely no change in voltage while the inducer was running. I've not removed the inducer, but there is no reason for any blockage and I blew through the tube and it was clear. The pressure switch works when the inducer is running. It seems like the control board is shutting the furnace down for no physical reason. I watched voltage throughout the process, and it runs between 27 and 25.9 volts the whole time (checked with two different meters... The new one is a Klein).

I will pull the inducer and make sure everything is still clean before I give up, but I have to know if this could be a problem with the board? Maybe a bad relay?
 
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Old 12-30-15, 08:48 AM
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on second thought

This new problem of the furnace running for four or five minutes, then shutting down and restarting one minute later sure seems like a bad draft safety switch. I replaced the original last time with an equally rated one that does not have a reset button. It was the only correctly rated one at the hardware store. (This place usually has everything, except apparently Carrier parts.) I assumed, since the new switch was cheap, it was a one use switch... expecting that if it goes off, it does not reset. Now I'm wondering if it works like a thermostat and resets when it cools? That would explain what is happening, although then I'll have to find why that switch is activating... maybe a weak reducer motor? The vent system is clear and everything appears to be working correctly.

Is there a way to test the inducer motor to prove it is running at the proper rpm?

Tonight, I'll jump the DSS and see if the furnace will run normally. That should prove whether or not that is shutting things down. If not, I'll do the same with the pressure switch. That will tell me if the problem is there and/or with the inducer motor.
 

Last edited by Tojose59; 12-30-15 at 09:08 AM.
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Old 12-30-15, 03:42 PM
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Jumped the draft safeguard switch and connected my ohm meter to the switch terminals. Five minutes into the run, the DSS opened. Furnace stayed running on the jumped wires. Good news is I know what is shutting the system down. Bad news is I have no idea why. The chimney has a cap so no birds can get in. I'll pull the inducer tonight but I expect it will be clean inside since it was apart a few weeks ago.

Before I pull the chimney apart, I'll order an OEM draft safeguard switch, but this generic one has worked fine for weeks.

My one last concern is the heat exchanger. I changed the baffles years ago because of a rust hole. Does anyone know of something like that causing this problem? The last time, the hole was near a limit switch, so it blew that constantly.
 
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Old 12-30-15, 03:59 PM
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I should think before I post. It can't be a problem with the heat exchanger. The inducer draws from the combustion side. Duh

So... If the vent (chimney pipe) is clear, and it's worked for years, then the only other possibility I can think of is could the inducer motor be running slow and I just can't see that? The bearings and the squirrel cage are fine. I can spin the motor by hand and it turns forever.
 
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Old 12-30-15, 05:16 PM
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Found rust flake build up where the chimney pipe turns into the basement from the outside wall (old house). About 60% blocked) Vacuumed that out and everything works fine now. Should have checked that first.
 
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Old 03-08-16, 12:05 PM
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Carrier Furnace Code 31

Did you ever figure out the problem on this? I have been experiencing the same problem for the last several months. I have had furnace techs out here 4 times. They have replaced the computer board, the pressure switch, checked voltage, checked ventilation and I think most of the things you mentioned. The last time they were here they adjusted the slope of the tube that carries the water away but that didn't make any difference. They are stumped and I'm frustrated. Any ideas? Thanks for any help.
Terri
 
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Old 03-08-16, 01:24 PM
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Welcome to the forums.

After you respond I'll move this to it's own thread.

Technicians don't throw parts at a furnace hoping to fix it. A competent HVAC tech should be carrying a manometer. This device gets connect to the draft inducer/pressure switch line and monitors the vacuum in the draft inducer. They should know immediately by watching the gauge if there is a vacuum/flue issue.
 
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Old 03-09-16, 12:30 PM
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Just as important, when the defect is cleared, the pressure measured by the manometer will go to a normal level.

You wont be guessing about whether you have solved the problem.
 
 

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