Carrier Infinity Furnace Troubleshooting

[64coupedeluxe]

Hi All,

Model # 59TN6A 2 stage 96%
IU (thermostat) SYSTXCCITC01-A
Software 131 493-14.02
Firmware 2.5.5.1.6.8
Natural Gas

Hi all,

This unit is just over 10 years old and never had an issue. I was getting a code 32 Low heat PS did not close or reopened.

If open for longer than 5 minutes, inducer shuts off for 15 mins before retry. If opens during blower on-delay period, blower will come on for selected blower off-delay.
Check for
Proper venting
Air leak between vestibule and blower compartment.
Restricted vent
Disconnected or obstructed pressure tubing.
Defective or miswired pressure switches
Excessive wind
Plugged condensate drain
Water in vent piping, possible sagging pipe
Defective inducer motor
Low inducer voltage (115v)

Not sure if this is just a coincidence but the infinity thermostat had not connected to the Carrier website/server for some months. This is an on going Carrier problem across all infinity thermostats. My heat had been working fine and then I noticed it was connected to wifi/carrier server. Well then my heat failed to work.

When heat is called for, the startup sequence commences. Gets to the inducer motor stage, which runs but goes no further. And just keeps running.

My early attempts to trouble shoot had me switching from heat/hold- how we always operate the system, to one of the pre set schedules. This got the furnace to run for 5 or so minutes. So then I switched back to hold and can't get it to operate again.

I cleaned out the condensate trap, it had little water in it but did have a very small amount of black grit in it. Cleaned all lines, no restrictions or problems. As far as I am aware, this style of trap does not need to be primed. Please correct me if I am wrong.

I removed the inducer motor and inspected the vent/******(known carrier problem). Totally clear and the motor housing is in good shape and clean.

Vacuumed both pvc vent lines and get really good suction. Got nothing out. Also put a fish wire up both from the outside for 25ft prior to vacuum.

HSI measured 46.5 ohms, which is in spec.

Purchased a manometer from amazon.

T'd in reading of .060 in hg. My pressure switches are rated at .30 and .75 in hg. Not sure if my cheap meter is reading right or not but it is lower than what the switches need. *Defective inducer motor is one of the culprits for code 32.

Disconnected vent elbow from inducer and got a reading of .119 in hg

I had also purchased new pressure switches as they were cheap and free return shipping and still had the same fault code. Replaced all vacuum lines with new vacuum line.

I then accessed the dealer/service icon on the thermostat. The fault code events are:

11/5 #31 High PS open 3 x

11/6 #32 Low PS open 3x

11/25 #31 HPS open 5x
11/25 #12 Blower on after startup 1x

11/26 #31 HPS open 1x

11/28 #32 LPS open 1x
11/28 #12 Blower on after startup 1x

12/4 #31 HPS open 9x

12/16 #32 LPS open 124x

12/16 #178 Indoor communication fault 5x

I forgot to mention that when the furnace first failed to provide heat I checked the Filter. It was on the dirty side and was due to be changed around now which is typically the time I get the yearly service. So I replaced it.

I was paying for the yearly service due to the warranty requirements from the installer/Carrier for the 10 year warranty. Which I'm pretty certain just expired a couple of months prior.

Code #31 High-Heat PS or relay did not close or reopened:

Check for:

Control relay may be defective

Gas valve mis-wired.

See status code #32

I watched a bunch of trouble shooting on Siglernorcal.com which is carrier tech training. So then I started to trouble shoot to see what voltages etc I had. Seeing I had in my mind about this all going **** up after it reconnected to the carrier server, I disconnected the IU (thermostat) from the wall to check my voltages. They all seemed good but when I reconnected the thermostat I got fault code 178 Indoor communication fault.

Thinking there was a break in my comms wires I cut back and reconnected. Same problem. Change to some spare conductors, same problem.

Took thermostat down to furnace and wired in short jumpers, same problem. Code 178

So I went into the Service icon, went to Installation and then hit Installation then Full Installation so that the IU (thermostat) could search for the indoor unit (furnace).

It could not find it after several tries and my only option was to select Demo Mode, which I can't seem to get out of...Help!

I have ohm'ed out both the board and the IU and they fall right around the carrier specs. Can supply these if req.

Voltage readings at ABCD connector(control board) with no UI connected

A-B 4.59VDC
A-C 4.6VDC
B-C .014VDC
C-D 28VAC

Voltage readings with C-D connected to UI but communications wires A-B disconnected from control board.

A-B 4.38-4.5VDC fluctuating
A-C 4.37-4.5VDC fluctuating
B-C 0.01VDC steady
C-D 28VAC steady

I saw one carrier video that stated during my second test with the UI partially connected that the first 3 readings should be fluctuating. Then watched another that stated they should be steady.

I saw mention to make sure that SW4-1 is turned off as this disables the communication lines, checked and was off.

Looked all over the board and nothing seems amiss.

At this point I feel like I'm nearly an authorized Carrier maintenance man But am obviously a ways off, but feel like I'm close to getting this worked out, but need some expert advice. At this point it's not about paying for a service call, it's that I don't want to be defeated by this mongrel. We have a fire place insert that keeps the house warm so it's not an emergency.

We have a NG hot water heater that is working fine and also gas cooktop.

Not sure what I am missing as from my voltage and ohm readings the IU should be able to see and communicate with the furnace.

How do I get out of Demo Mode and what should my next troubleshooting steps be? Thanks for reading this far and any suggestions you can offer.

 

[64coupedeluxe]

So I just looked back on a few copies of the service records I have. Back in 2016(installed 2014) there were over 40 faults for pressure switch related problems and it stated pressure switch should be swapped out. I do now recall that they swapped out a part under warranty.

The service record in 2022 again states low pressure switch problems and drainage may be an issue. I never really looked at this before as the techs have always told me that everything was ok.

This looks like it has been a recurring problem and due to it not being installed in a 100% correct way.

 

[Buckaroo5]

You have dug in pretty deep. Although I am a data guy, your post was making my head hurt. I will go back at read it more thoroughly.

This fall I was having code 31/32 issues and started this thread - perhaps you already ran across it....

Carrier Furnace Code 31 and Short Cycling

I posted this on the tail end of a similar, older post but did not get much response so trying again..... Turned on my son's Carrier Infinity 96 Gas Furnace (Model 59TN6A) first time this season and would not fire up - code 32. Poked at it a little bit and it fired up but noticed it was...

www.garagejournal.com

I had never cleaned out the condensate trap and lines so did that first thing - remember to make sure the drain from the trap is running clear. I think cleaning the condensate trap & tubing was the ultimate solution although it took a day or two to resolve due to, I believe, residual moisture in the tubing. I was going down the route of replacing the inducer fan but wanted to test first and was looking at buying a manometer - I decided that the inexpensive digital manometers did not have sufficient precision to measure at the levels I needed. I was looking at purchasing a manual Dwyer Flex Tube Manometer when the fault went away and the furnace stopped short cycling. Has continued to operate correctly so far and I continue to monitor it for error codes. I did note that the inducer fan on my model has been redesigned by Carrier and I verified I still had the original design which is a known source of problems. This video helped me and it describes how to determine if you have the old design....

Also, interesting to note we had to replace that fancy thermostat a couple of years ago, too. Luckily the furnace was just inside of warranty at the time because it is pricey. I don't remember exactly but I think it was a communication issue. This code 31/32 issue occured recently, a couple of years after the thermostat replacement. The new thermostat is working fine.

 

[Buckaroo5]

OK, I went back and reread your post. I am sorry but I can't help with the thermostat issue - the Carrier tech replaced ours which was an easy decision for us because it was under warranty. Perhaps all those historical pressure sensor faults could be related to water/condensate. You mentioned the ****** on the inducer fan and I do believe that the redesign made that larger. As I recall, there are youtubes out there where folks have drilled out that ****** but I was not going to go that route myself. I was just going to replace it before the issue cleared up.

 

[64coupedeluxe]

You have dug in pretty deep. Although I am a data guy, your post was making my head hurt. I will go back at read it more thoroughly.

This fall I was having code 31/32 issues and started this thread - perhaps you already ran across it....

Carrier Furnace Code 31 and Short Cycling

I posted this on the tail end of a similar, older post but did not get much response so trying again..... Turned on my son's Carrier Infinity 96 Gas Furnace (Model 59TN6A) first time this season and would not fire up - code 32. Poked at it a little bit and it fired up but noticed it was...

www.garagejournal.com

I had never cleaned out the condensate trap and lines so did that first thing - remember to make sure the drain from the trap is running clear. I think cleaning the condensate trap & tubing was the ultimate solution although it took a day or two to resolve due to, I believe, residual moisture in the tubing. I was going down the route of replacing the inducer fan but wanted to test first and was looking at buying a manometer - I decided that the inexpensive digital manometers did not have sufficient precision to measure at the levels I needed. I was looking at purchasing a manual Dwyer Flex Tube Manometer when the fault went away and the furnace stopped short cycling. Has continued to operate correctly so far and I continue to monitor it for error codes. I did note that the inducer fan on my model has been redesigned by Carrier and I verified I still had the original design which is a known source of problems. This video helped me and it describes how to determine if you have the old design....

Also, interesting to note we had to replace that fancy thermostat a couple of years ago, too. Luckily the furnace was just inside of warranty at the time because it is pricey. I don't remember exactly but I think it was a communication issue. This code 31/32 issue occured recently, a couple of years after the thermostat replacement. The new thermostat is working fine.

Thanks for the link. Had not seen that particular video. Too bad the audio isn't great and he doesn't really go into too much detail in regards to troubleshooting.

My inducer motor already has the X. It was also bone dry. Not saying there isn't something wrong with it. In the comments that guy stated these motors often have a seal failure around the shaft which causes low output. Though he did not state if thats the X/non X motor or not.

All drain/vent lines were vacuumed/blown clear but had little in them, when I pulled them apart. The motor that guy pulled off had water sloshing out of it.

All that is a moot point seeing I now can't get the UI(thermostat) to talk to the furnace.

 

[WildBill]

I had somewhat of the same kind of issue and water had backed up into the fan. I first cleaned and drained the trap which didn't fix it, but then spun the fan by hand and could feel the water down in the bottom of it. Once I got that out it was ok.

I also had a different never ending issue with lots of pressure switch errors that was resolved by putting in a used mainboard. Everything measured OK but it just kept getting random errors. I lucked out and found a place on ebay with good reviews that has tons of used carrier parts. Was only $18 and I haven't had an issue since.

 

[64coupedeluxe]

I had somewhat of the same kind of issue and water had backed up into the fan. I first cleaned and drained the trap which didn't fix it, but then spun the fan by hand and could feel the water down in the bottom of it. Once I got that out it was ok.

I also had a different never ending issue with lots of pressure switch errors that was resolved by putting in a used mainboard. Everything measured OK but it just kept getting random errors. I lucked out and found a place on ebay with good reviews that has tons of used carrier parts. Was only $18 and I haven't had an issue since.

Thanks.

If I could get a used board for $18 I would certainly try it but I would also like to get to the bottom of the fault to make sure it doesn't potentially damage any new parts.

 

[Buckaroo5]

The thermostat issue and the code 31/32 issue may be independent. Are you thinking they are related?

 

[64coupedeluxe]

The thermostat issue and the code 31/32 issue may be independent. Are you thinking they are related?

I have no idea really but from research I would say they are separate.

But right now I can't trouble shoot the 31/32 because it has a communication fault 178 and would do nothing. And now I have it in the "demo mode", which is all in my first post.

 

[Buckaroo5]

Stupid suggestion - try shutting off power to the furnace and letting it reboot. If that doesn't do it, leave the power on and disconnect the thermostat from the wall and plug it back in again.

 

[brewchief]

You should be able to disconnect the communication wires to the UI and simply jumper r and w to test heat operation and troubleshoot that part of the problem.

 

[64coupedeluxe]

You should be able to disconnect the communication wires to the UI and simply jumper r and w to test heat operation and troubleshoot that part of the problem.

The UI connects via an ABCD molex plug but it does have a normal terminal block. I was trying to find my old systems thermostat to try wiring it in.

I will try the jumper as that's easy enough.

 

[64coupedeluxe]

You should be able to disconnect the communication wires to the UI and simply jumper r and w to test heat operation and troubleshoot that part of the problem.

That worked .......Now what?

The furnace kicked on and I let it run for around a minute. I had the manometer T'd in and it was still showing lower pressure than it should, so I was surprised that it did.

I have ordered a more expensive manometer that was recommend by some hvac guys online, so I can see what is really happening.

Seeing it fired up and run does that prove my control board is ok?

And I guess it would prove my UI is either defective or I just need to try a full install?

 

[64coupedeluxe]

So I did the full install which kicks the UI into searching for the indoor unit, which it can't find.

 

[hobie18]

There are some mercury switches available...

Not a carrier expert. The line from the fan to the switch/sensor is clear, connected, and sealed?
Fan is fully seated and sealed to base?
Can you redo the thermostat or swap it out with a simple one? Leave it running for a longer time.

What type of filter is in place? Remove. Try easier one.

 

[64coupedeluxe]

There are some mercury switches available...

Not a carrier expert. The line from the fan to the switch/sensor is clear, connected, and sealed?
Fan is fully seated and sealed to base?
Can you redo the thermostat or swap it out with a simple one? Leave it running for a longer time.

What type of filter is in place? Remove. Try easier one.

Not sure what you mean about mercury switches?

By lines do you mean the vacuum tubing? If so I stated I installed new vacuum tubing.

What do you mean by redo the thermostat?

Yes the inducer motor housing is seated correctly.

I can't seem to find where I stashed the old dumb thermostat from my prior system, but as I understand it by jumping the W-R wire I am doing the same thing.

It's a 4" brand new filter. Same filters as I've always used.

The furnace starts and runs fine with the jumper in place.

 

[64coupedeluxe]

I found info regarding the Model Plug which these boards have. It is right next to the molex plug and I was thinking perhaps I bumped it. The board needs this for communication. It has a couple of resistors, and on the back of the cabinet door it states resistance value between the plugs.

So I pulled that off and tested it. Unfortunately it came back in spec. Plugged it back in and was hoping that it would all start working together. No such luck.

There is a guy local to me on fb marketplace that has the same thermostat for sale, just waiting for him to get back to me.

 

[hobie18]

Dumb thermostat or jumper works. Great. Hopefully it will run for a long time. if not then other techniques. Yes I saw you change the tubes.

 

[64coupedeluxe]

Dumb thermostat or jumper works. Great. Hopefully it will run for a long time. if not then other techniques. Yes I saw you change the tubes.

 

[64coupedeluxe]

Not sure what went on with the above reply...

Anyway, I connected a simple on/off switch where my thermostat lived. I have run a few 10-20 min cycles and the furnace is running fine and completes the heat cycle correctly with the blower fan running for a period of time to cool it off.

There are no codes flashing on the control board, so due to that fact, plus I get the correct readings between the A-B-C-D pins, I am going to hope it is just the thermostat.

I will update the thread as to its conclusion for future troubleshooters.

This is the 3rd forum I have posted to, asking for help with this problem. Actually you guys in this forum are the only ones to have helped Garage Journal for the win again.

HVAC guys sure don't seem to like DIY guys and I'm sure they all take their vehicles/equipment etc to a dealer for any work.

 

[hobie18]

Only question that may arise; if it fails after a long interval. Seems unlikely. Thermostat seems the problem. If not, the unit could be resetting itself, and making us think it is the thermostat. I have seen that many times.
Think you are on the right track. Well done.
Thank you for updates!

 

[64coupedeluxe]

Only question that may arise; if it fails after a long interval. Seems unlikely. Thermostat seems the problem. If not, the unit could be resetting itself, and making us think it is the thermostat. I have seen that many times.
Think you are on the right track. Well done.
Thank you for updates!

Thanks mate.

I'm going to run it for longer tomorrow instead of heating with the wood stove, so with luck that will narrow it down. Though the 32 fault code had been consistent through a few days of troubleshooting and switching the unit off etc. The more expensive manometer is turning up tomorrow so I will be able to get a better idea of what's going on with the inducer motor.

Not sure if I'm out of the woods yet but I've certainly gotten a better grasp of what should and shouldn't be going on with the unit.

I still plan to get my yearly service so they can check the combustion and whatever other safety related items they check.

Couldn't meet up with the local guy with the same stat for sale. It's only $50 but I'm actually inclined to buy the latest model from ebay as I can get it with a 3yr warranty for around $400 delivered. And it doesn't seem like they support the older model anymore with updates. 10 yr old paperweight.....

To get the carrier 10 yr warranty I need to buy and then have installed from a carrier dealer, which would be pretty close to $14-1500, from what I've seen online.

 

[64coupedeluxe]

She has been running fine through the on/off switch for the last 3 days. No codes are flashing on the control board.

Must be the thermostat.

 

[dscheidt]

She has been running fine through the on/off switch for the last 3 days. No codes are flashing on the control board.

Must be the thermostat.

Can't you use another thermostat? Worst case, you can hook a heat only two-wire stat up the same way you have your switch.

 

[64coupedeluxe]

Can't you use another thermostat? Worst case, you can hook a heat only two-wire stat up the same way you have your switch.

I could use another thermostat if I had one. Wasn't going to go buy a cheap thermostat just to run the furnace for a few days as the switch does the same thing, albeit with me watching the temperature.

 

[todd_fuller]

Not sure how computer savvy you are but there are a couple projects out there to talk to the furnace on the ABCD bus. Might give you a clue whether you need a new thermostat or not.

GitHub - bvarner/gofinity: A library for interacting with the Carrier Infinity / Bryant Evolution communicating residential HVAC systems

A library for interacting with the Carrier Infinity / Bryant Evolution communicating residential HVAC systems - bvarner/gofinity

github.com

I had a firmware update kill my WiFi connectivity and found you can manually force a firmware update even to old versions if you suspect an update is causing the comms error.

 

[64coupedeluxe]

Not sure how computer savvy you are but there are a couple projects out there to talk to the furnace on the ABCD bus. Might give you a clue whether you need a new thermostat or not.

GitHub - bvarner/gofinity: A library for interacting with the Carrier Infinity / Bryant Evolution communicating residential HVAC systems

A library for interacting with the Carrier Infinity / Bryant Evolution communicating residential HVAC systems - bvarner/gofinity

github.com

I had a firmware update kill my WiFi connectivity and found you can manually force a firmware update even to old versions if you suspect an update is causing the comms error.

Thanks for the info and link. I did come across that in my searching. I'm not that computer savvy and don't want to put the hours in to learn what to do. I do have a background in electronics from a long time ago, which has helped through the troubleshooting, but I have seen the latest thermostat version on ebay for $300 brand new.

I have seen a lot online, about problems with carriers thermostat. It would be great if some computer genius made something to take over the carrier junk, and made it easy for the average diy guy to install.

I have found a local hvac guy through my wifes real estate connection. I spoke to him at length of everything I had done and found. In talking to him I could tell he really knows his craft and he owns his own small business. I have him coming out on Tuesday to do my yearly service check and to look everything over.

I will see if he can re-install the thermostat otherwise I will get the latest model and install it.

 

[64coupedeluxe]

System has been running fine and then christmas eve code 31 HPS switch started flashing. Even though it was flashing the furnace was still running fine.

Some of the vent piping looks to be on the margin of having enough pitch and the condensate drain line barely has any pitch. I go outside and hear gurgling on the vent line, which was the first time I've ever heard that. Looked at the pipe with a flashlight behind it but could not see any sign of water.

From my calculations it also looks like my pvc run is getting close to the maximum allowed.All of this explains why my system has had these recurring faults over the years.

The furnace is still running fine and the code 31 and gurgling is gone. Will update once the hvac guy comes.

 

[danski0224]

Some of the vent piping looks to be on the margin of having enough pitch and the condensate drain line barely has any pitch.

I was going to suggest taking a really strong look at the vent piping, with the installation instructions and tables in hand.

Also had an unusual problem with a pressure switch that had an internal air leak. Pressure switches are normally durable, but if they aren't too expensive, it may be cheaper/easier to buy a new one vs expensive diagnostic equipment.

 

[64coupedeluxe]

I was going to suggest taking a really strong look at the vent piping, with the installation instructions and tables in hand.

Also had an unusual problem with a pressure switch that had an internal air leak. Pressure switches are normally durable, but if they aren't too expensive, it may be cheaper/easier to buy a new one vs expensive diagnostic equipment.

One of the first things I did was replace the pressure switch. No change.

From my manometer readings the hvac guy thinks the inducer motor or its capacitor are showing signs of age. And that, with the possible venting problems is now not able to overcome the less than ideal installation.

 

[danski0224]

One of the first things I did was replace the pressure switch. No change.

From my manometer readings the hvac guy thinks the inducer motor or its capacitor are showing signs of age. And that, with the possible venting problems is now not able to overcome the less than ideal installation.

Interesting.

The capacitor should be able to be tested, right?

If the end of the motor isn't totally enclosed, maybe the RPM can be measured.

Does the inducer motor have a capacitor, or is it one of those fancy and expensive ECM motors?

Maybe the poor venting (assumed on my part) is causing the motor failure. I have noticed that Carrier has much less leeway in their venting tables.

 

[64coupedeluxe]

Interesting.

The capacitor should be able to be tested, right?

Maybe the poor venting (assumed on my part) is causing the motor failure. I have noticed that Carrier has much less leeway in their venting tables.

I don't have a multi meter that can test capacitance.

 

[danski0224]

I don't have a multi meter that can test capacitance.

That would be cheaper than a fancy manometer, methinks...

I would be surprised if the capacitor is failing.

My gut tells me that it is a venting installation problem.

 

[64coupedeluxe]

The hvac tech tested the pressure switches and the inducer motor and stated the motor was in spec, switches fine. In the vent tables I have around 6-7ft under the limit for 2" pipe.

Instead of going with a 20ft stick of pipe and 1 join for a short section the installers used 10ft sections so an extra joint. It might be deceiving to the eye but the pipe looks like it dips slightly at the joints.

Also along the length the pitch was up and down.

Besides all this the furnace has still been running fine on my off/on jumper switch, with no codes. Not heard anymore gurgling.

The tech had a spare UI and he dropped it off to me the following day for me to hook up and have everything running via thermostat. Great guy to do this....

So I hook it up and it connected to my furnace and I did the install. So my OG thermostat is bricked. I call for heat and the furnace starts to run through its process. After the inducer spins up the board starts flashing code 32 again FFS! And will not fire.

Haven't called the tech again as it was new years eve and then day. Will call him tomorrow.

So I adjust the vent pipe as much as I could to make the pitch better. Still get code 32 with the UI connected. Disconnect UI and go back to my switch jumping R/W wire. Furnace runs until I turn switch off and no codes. This makes no sense to me. I have done both scenarios with my old pressure switches and also the new ones and there is no change.

Then I remove the condensate drain pipework and put a small section of hose going straight into a bucket, to ensure that the line which only has a small pitch is taken out of the equation.

Same thing, won't work and code 32 on UI but on my switch it runs fine until I turn it off with water happily trickling into the bucket.

I don't understand how it will run on my switch and no codes but when I connect the UI it will not fire and pulls code 32???

I know of the service bulletin about the vent ****** on the inducer motor. My motor is the replacement/upgrade with an X marked on it which has a bigger diameter ******. Even though anytime I checked this it was clear and I have seen no evidence of condensate in the inducer motor, I drilled the port out larger....no change.

At this stage I'm thinking I will buy a new inducer motor as it's around $200 plus shipping.

 

[hobie18]

Mom's is acting up again. Carrier. I will find out the number. Thought we solved it with the easier filter. I do lnow they had to donsome worl when we got rid of the fancy thermostat and went with a cheapie.

Want to hook up a simple thermostat mercury switch? Avoid having to jumper on and off?

 

[64coupedeluxe]

Mom's is acting up again. Carrier. I will find out the number. Thought we solved it with the easier filter. I do lnow they had to donsome worl when we got rid of the fancy thermostat and went with a cheapie.

Want to hook up a simple thermostat mercury switch? Avoid having to jumper on and off?

No I don't want to use a normal thermostat as I will lose some benefits and efficiency of my 96% unit.

 

[danski0224]

Are the codes displayed on the thermostat or in the furnace?

 

[hobie18]

No I don't want to use a normal thermostat as I will lose some benefits and efficiency of my 96% unit.

What are those? Could that be a clue?

 

[64coupedeluxe]

Are the codes displayed on the thermostat or in the furnace?

With the thermostat connected the code flashes on the furnace and I can see it in the "service" icon. With only the switch connected it does not show a code on the furnace and it runs for as long as I want it to.

Briefly texted with the tech today and he stated the thermostat he lent me must of been bad, and if it is still running fine on the switch that the furnace is fine.

I find it hard to believe that there is nothing going on with the control board seeing it's the 2nd thermostat to not work. But everything I find online does state if the furnace is running fine when R/W jumped, then it's a thermostat problem.

I also cut my siding and raised the exhaust pipe higher. It now has the factory spec 1/4" drop per ft.

When I have the switch in place (R/W) I see 27V on 1 terminal of the low pressure switch and then see it on the other terminal when it closes and then it commences to fire.

With the thermostat on the molex plug I never see the 27V, the inducer just keeps spinning and it never starts.

 

[danski0224]

Interesting. I am not familiar at all with Carrier Infinity stuff.

Apparently, the thermostat/controller is processing something during normal operation.

When I have the switch in place (R/W) I see 27V on 1 terminal of the low pressure switch and then see it on the other terminal when it closes and then it commences to fire.

With the thermostat on the molex plug I never see the 27V, the inducer just keeps spinning and it never starts.

Does your tech have access to another system that your thermostat could be tested on?

If the system runs with the thermostat jumpered out, then that should eliminate the whole furnace. If something was wrong, then the safeties would take over and prevent operation.

That leaves the thermostat/controller, the part of the main board that talks to the thermostat, and its interconnecting wiring.

Thermostats are generally durable. Two thermostats with the same issue at the equipment seems awfully suspicious to me.

I would look very, very closely at the Molex connector pins and see if any of them have been pushed out or are otherwise misaligned. That includes pulling on the wires at the connector to see if something is loose.

I have had 2 Carrier junkers with bad pins from the factory. It took a lot of diagnostics with tech support to narrow down the problem. The quoted part above is a clue to me.

Your tech may have to spend time on the line with tech support.

I would also read the instructions and see if twisted and/or shielded wire is specified for the thermostat, and the specified gauge.

I get that it has been working,,, but you never know.

Otherwise, whatever is interfacing the thermostat to the furnace control board could be bad.

At this point, without access to some very in depth information, one is stuck swapping "known good parts".