first remove the rubber tube from the barbed fitting at furnace end...use a paperclip and poke the fitting clear. Then make sure the drain from the furnace condensate water is not blocked.
Check those 2 items first and let me know.
I remove the hose from the furnace side where the other end goes to the collector box side of the pressure swich and stuck a paperclip though it and worked it around, replaced the hose and restarted the furnace. Switch still not closing. I removed the drain hose off the condensate collector box on the side of the furnace and do feel air blowing out of that while the fan is running. Hose from that box to the pump-out box is free from obstructions.
Remove the intake pipe from the furnace and leave just the exhaust pipe hooked up.
Then try unit with intake pipe off.
This is a sign of the following:
Plugged secondary heat exchanger - this requires attaching a shop vac to the drian applying suction.
Or pressurizing system from the vent side and blowing and then reverse and apply suction.
Make sure the rubber tubes do not have cracks in them.
The switch is to prove air flow through the furnace before ignition can happen...if anything in the exchanger, tubing, venting etc cause too little air to draw on the switch, then 31 appears.
You have 2 tubes on the switch right?
One from burner box and one from exhaust?
did you clean that fitting out ?...the one over the condensate?
Thanks. Yours is a bit more stubborn than most...you will need to see if the secondary is plugged. Or have a tech check for you.
This happened often eneough to where they had to switch 20 year to lifetime warranty on exchanger replacements.
If plugged and not able to clear, the exchanger (parts) are under warranty.
They asl issue 400 dollars to the contractor towards the changeout. Customer final cost is around 4-5 hundred.
you have a primary heat exchanger...and then a second one...this is how we get 90% instead of 80% efficient. The second exchanger is where the water condensation is made. It can get plugged with "thickening water" and if unit was installed with an one inch slope to front the unit can get plugged.
You should not feel pressure coming from burner box..but you should feel suction coming from other tube port above condenste drain.