Carrier Furnace Code 31 and Short Cycling

[Buckaroo5]

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 flashing code 31 and was short cycling (3 minutes). Cleaned out the condensate drain, trap and verified all the small tubes were clear. Cycling now at 4 minutes but not sure that increase is meaningful. Verified vent line (short run w good slope) was clear of debris and wasp nests using endoscope. Measured voltage on output side of high pressure switch and could see it closed properly when inducer fan started and was closed throughout the cycle - still closed when the burner cut-off.

Apparently Carrier redesigned the inducer fan assembly and I see on the interweb that many have been replaced associated with code 31 although interestingly the code description does not mention it as a possible cause. I don't have a manometer to check it directly. If the high pressure switch is closing and staying closed (which I think it is based on my electrical testing), could the inducer fan still be the problem causing the code 31 and short cycling? Considering buying a Dwyer U Tube manometer ($70 or so) to verify if inducer fan is the culprit if I can't feel good about it needing replaced as the new part is $280. I see a lot of cheap digital manometers on Amazon ($30 or so) but I have trust issues.

Thank you for reading.

 

[Crazyjake8493]

Without a manometer, there's no way to accurately test draft. I have a Testo, not sure how the cheapo units on Amazon are. That risk is up to you. I think I'd rather have a Dwyer than a cheapo - just personal preference. A Testo or Fieldpiece should still be cheaper than a service call, even after adding the cost of a part.

I had a problem with our Goodman a few years ago, kept losing flame and refiring, so the cycles were very long before reaching temp. The draft tested good, but so did the pressure switches. Turned out to be bad contacts inside one of the pressure switches. When I put the low-fire pressure switch on my Fieldpiece Draft Simulator it looked good but but occasionally the light on the simulator would flicker. Not even enough to read on a halfway-decent multimeter, but enough for the furnace to shut off the flame and restart the cycle.