I have tried almost everything in my arsenal to try and diagnose the problem. First off I have a Carrier Gas Furnace that is almost 7 years old. Model 58MCA-80 (See attached photo for more information), with pressure switch #HK06NB123.

Intermittently I get a Red Flashing Code #31 when the unit tries to cycle. So far I have tried the following to fix


Blown out the drain line
Blown out each of the vacuum lines connected into the pressure switch
Replaced the pressure switch with model HK06NB124 (Per my local Carrier Service Center), this did not fix the problem so I have since re-installed the existing old switch
Electrically tested the pressure switch. I think I briefly got 25V when the unit called for air. I did this by touching my voltage meter to either electrical connector one the pressure switch itself.
Jumped the pressure siwtch by connecting the yellow and orange wires. When jumped the unit will run correctly until I disconnect the wires.
Checked both the exhaust and supply air piping.....all seems ok
Inducer motor spins fine.....although I do not know how to test it to make sure it is spinning fast enough?


After all that I do have one random solution that will temporary let the unit work. If I disconnect the upper vacuum hose from the pressure switch (Attached photo with red arrow) while the unit first turns on, it will cycle and start burning gas like it should. I then reconnect the hose and everything is fine for the next 3-4 hours or so.

Let me know if you have any help for me! I am almost at the point of calling a service tech. to help me out.

Thanks in advanced!

I'd recommend you replacing the draft inducer motor and wheel blower. It sounds like the pressure swicth is senseing the backflow initially and your good, but after use it doesn't. You've narrowed it down to the IDM, let me know how it goes. I'm ordering one for my unit.

Garekare,

Thanks for the response, luckily for me the problem was actually much simpler. It ended up being a combination of two things, so not for sure which was the actual culprit.

1. We removed the condensate drain trap in the lower access panel and cleaned it out with water. When we flushed this out, lots of black/grey crud came pouring out which could have led to an intermittent shut-off of the system.
2. We removed the upper air intake pipe and after even further inspection, found a little brown leaf in the pipe.
3. We also cleaned the flame sensor with some steel wool to remove any oxidation.

All is fine so far!

Hope this information helps someone else out. I guess in hind sight, it is better to do a very in-depth cleaning job rather than try to diagnose broken switches or motors.

Thanks,

-Rex Buchanan

GREAT POST!

Just saved me in NE IL... it is -16F today and my furnace did the same thing.

Was able to restart by pulling the upper hose and get my heat back on.
I will check the drain when the house gets somewhat warmer. Only got down to 64 thanks to your post and pics.

Very well done Rex!

Dean in IL

OK... found my problem.

It was not the drain box.

About 3 hours after I pulled the top hose, the house had warmed to 66... however, the main blower was running, but there was no heat and the burners were off..

The board showed a code of 33.
I shut it down and pressed the reset on the burner box and it 'clicked' back to reset position. The high temp had triggered.

This led me to a possible bigger problem so I shut off the switch at the furnace and went into the -17F to check the pipes to outside.

The exhaust had a big icicle hanging to the ground... not a big deal... break it off and go on.

The intake which has a 90 degree turn-down was packed with snow! WTH?
I cleaned the elbow section with a plastic spoon and got out a bunch of snow / frost.

Went back in, rehooked the small vent tube, flipped the switch, and voilà, it works normally again with everything hooked up.

Thanks again for the posting Rex and Garecare!

I kept getting a code 31 when the outside air would get close to 0. I opened the bottom panel on the furnace and disconnected the rubber hose from the condensation trap. At first no water came out, I spun the blower mother by hand a few time and then it drained about 5 cups of water. Idk where it was holding all of that water. Anyway it works fine now.