Trane XV80 Code 3 three Flashes

Old 02-20-18, 09:58 PM
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Trane XV80 Code 3 three Flashes

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What I know in plain test>
I need help with a furnace: Trane XV80 TUD120R9V5K4 that consistently flashes a code 3 Pressure Switch Failure.
I have replaced the switch with a replacement with a similar number or is said to be the correct replacement.
We have been without furnace heat for 5 days and its cold here. We have been using the oven and a small space heater to keep the house above 50 degrees F. I ordered a pressure switch from a internet HVAC house they claimed the switch was a direct replacement for my switch. The labeling was consistent except for the Inch of water rating on the original switch was labeled .41 inches of water column and the new one has two range values. Since the pressure switches are both dual switches.
The new switch was SWT02357. The original Trane Part number BA20107 is definitely bad you can suck gently on the port and air will leak in the diaphragm has a hole in it. I know that .41 inches of water is a very low PSI pressure. 0.014 psi.The diaphragm can be damaged very easily by blowing or sucking on it too hard. You can easily produce up to 15 PSI by blowing with your mouth. Be careful or better yet use a monometer. They want $400 for a good one. You could use a piece of hose in a U shape with some water. as a measuring device.
You can test the switches by connecting a multi meter and blowing gently on the port.

The new switch works because I was able to get the furnace to run for an hour by gently blowing on the port hose for a few seconds while the furnace was trying to start and quickly moving the hose to the burn chamber port. It ran until the temp was met and then shut off and won't start again. The Control board flashed a code 3 the full time it ran and is now locked out again.

Is it true you have to switch off on off on off on to reset the control board? (to get it out of lockout).
Is the function of the two pressure switches just used to verify the burn chamber is clear of ignitable gasses and verify the flue is clear of obstructions? This is a Question???
What is the normal burn chamber Port pressure?
Is it a pressure or vacuum on the burn chamber port connected to the pressure switches?

My guess its a pressure not Vacuum.
Does the Draft fan run at Two Speeds and if so what should the pressures be?
If it Runs at two speeds, what order is the Start Process.

Call for heat
clear combustion chamber Timed verified by pressure switch.
speed up draft fan verify increased pressure Flue clear
Start igniter heater timed
Open gas valve
Check flame sensor if no flame shut gas valve
There are three flame sensors one conduction type and two IR flame sensors.
If the flame sensors are all good the furnace runs.
This is my understanding of how it works.

One switch appears to be connected to the gas valve and the other to the control board.
I also know that a bad or worn-out draft fan can cause a failure here. The fan doesn't appear to be bad but I have ordered one and it will be here on Thursday (in two more Days)
You might ask why I don't just call an HVAC company. Well I did and they quoted me $300 to just show up. We are about to retire and on a fixed income. I am a electronics and Automation Engineer by trade and should be able to figure this out.

My concern it there may be a hole in the heat ex-changer but I don't think this is the case.
I recently removed a tank water heater and I have not closed off the Teed off 4 inch flue.
The furnace has worked fine with the open water heater flue for months after we put in the tank-less water heater and took out the regular type water heater.

Need some help understanding the reason for dual pressure switches and why they have different pressure settings.
Why the furnace won't start. I think the pressure switches on closing or opening at the right times is the problem.
The flame sensors and igniter all work. It has to be a pressure issue.

Do HVAC people regularly adjust these pressure switches?
Please Help if you know the answers to these questions.

But what is the problem?
Old 02-21-18, 06:41 PM
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A lot of "stuff" here.

I have never replaced a pressure switch. They are called pressure switches but they actually run under a vacuum. It's almost impossible to diagnose a vacuum issue without a basic gauge. Even a water column device can be effective.

First thing to check is to remove the line from the inducer and clean the little tiny orifice at the bottom of the fitting. A small drill bit, pin or piece of wire can be used. It needs to be clean into the actual blower.

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