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Having trouble with the pressure switch .. Ill give a brief history on this unit which may help in diagnosing this problem . Unit installed by professionals in 1996. Only had one repair also by installers ..a new blower motor in 2007. At that time the tech showed me how to do a little preventive maintenance by cleaning up the inside of unit with a shop vac and checking the vacuum/condensation tubes for pinches, algae, and damage.. I would usually shut down power to the unit from spring to fall and do this minimal maintenance . Each year the unit would take 3 to 4 stop start cycles to get the burners to light off . I figured this was just due to the fact that gas (propane) had not been run through the system for a while similar to RV gas appliances.
So I never had to deal with the codes before.. No trouble for 19 years

So this year it was the same ..I fired it up in September and all seemed well til night before last when I woke up to a cold house after lowering the thermostat to 65 In the morning I investigated and found a code 31.

After reading the owners manual I now know that its the pressure switch and that 95 % of the time it is an airflow problem .. So this is what I have done and found ..

Yesterday. I Checked both filters up stairs and in the unit. Checked the intake and exhaust ABS pipes for wasp nests and obstructions . Clear as far as I could see . I tried start stop and it came on after I tapped the pressure switch housing with my fingernail. After running for a while. I stop/started it several times and all was well. So she ran fine all day set at about 70 degrees ...I lowered it to 65 for the night and she was code 31 again this morning ..

So today I vacuumed out the ABS from the outside, I disconnected and checked the tube from the inducer to the pressure switch ...clear, Disconnected the small tube from the condensate trap and gently blew some water thru it to see if any dirt or algae came out...clear..I rerouted the same tube so it didn't have any rising sections (to prevent possible double trapping) Checked the electrical leads to the pressure switch ...ok ...Started it up and it has been running fine again ..

So here are my questions Since it seems to operate fine when it is warmed up, but goes code 31 on me when its cold start what is the next thing I should do?. Before I start taking the trap off and the ABS pipes inside the unit to check for obstruction is there anything that the cold vs warm start problem points to?

I realize that I may be going after this all wrong ...please help
 

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Clean the trap.

Also check for blockage at the port the pressure switch hose attaches to the inducer.
 

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1) Where the intake pipe attaches to the burner box on some of them there is a plastic screen inside which can get debris in it. Don't ask me why the temp difference is an issue as it may be a co-incidence.

2) One of the hoses from the pressure switch or trap goes around underneath the inducer and attaches to the collector box it is mounted on. The port it attaches to can get plugged and sometimes you have to yank the whole inducer off. Huge pain in the azz and then you need red RTV silicone to fix the gasket.

3) Not uncommon for the pressure switch to fail from vibration

4) A lot of those units had the secondary heat exchanger rot out and there was a class action lawsuit. Google: Carrier class action lawsuit furnace heat exchanger. Need a Carrier dealer/tech to change it if faulty.

5) The pressure switch should be checked with a manometer to see if there is enough vacuum. The inducer may be slowing down and there is not enough vacuum or the secondary heat exchanger has holes in it.

Lots of reasons you get that code. Ideally you want to check the vacuum first.
 
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