Trane VX95 - no heat, no ignition, no gas smell


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Old 10-23-21, 01:35 PM
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Trane VX95 - no heat, no ignition, no gas smell

I have a Trane VX95 HVAC system that was installed in 2016 and manufactured 12/2015, model number in the heating portion shows TUN2B080A9V3VBA. I fired it up earlier in the week and got no heat out of it. Getting a code 2 on it. After it cycles I want to say 3 times through the whole startup sequence the inducer fan stops then the main blower motor starts running on one of the lower speeds. The 5A fuse on the control board is good.

Here are the things I've done to troubleshoot it so far:

I have pulled and cleaned the flame sensor (it didn't look bad at all).

I made sure there was no water in the trap and all the drain lines were clean and clear and no problems with flow through them. I even bleached the trap just to make sure and blew out the lines and the trap.

I made sure that I am getting vacuum on the vacuum lines (there isn't much at all though) and that the lines were completely clean and clear with no blockages.

I checked for 24 volts at the transformer, and it was putting out about 27.5. I work with low voltage equipment access control, intrusion, and CCTV along with all the programming and such as a lead field service technician with Honeywell HBS, so this is pretty normal to see a reading of this to me.

I am getting 51 ohms resistance on the igniter. I do not have a good view of it through the little window so I can't tell you if it glows or not since I have to run upstairs to turn the thermostat to heat so I can get a heat command to the unit, and from my understanding I will not be able to get back down in time to see it glow anyway.

The inducer motor spins up as normal and some time in the cycle and I get voltage on the gas valve for a number of seconds, then things spin back down and it tries to start the cycle again. I hear relays kick in while this is all happening.

Thermostat seems to be working as normal, and the air conditioning side works no problem.

One thing I did note is that when I pulled the plug off of the gas valve, a White Rodgers 36J54-202 with Trane part number D674542P01 I get a bit over 5 million ohms of resistance between the high and common, and the low and common terminals. I get infinite resistance on between the high side and the low side terminal, which I would expect. When I troubleshoot wiring and circuits at work and I see resistances that high I generally say the wiring or whatever device is bad because it is pretty well infinite resistance. I simply do not know enough about troubleshooting this component to say anything on it, and I couldn't find the specs for resistance on it.

I haven't troubleshot the pressure switch because I am not 100% sure what that would involve. It is a Honeywell C342634P62. I am sure that I could borrow a manometer if needed to troubleshoot it since it is a vacuum actuated switch it looks like.

My question is where do I go from here, and what would you say is my problem based on what I have listed? Thanks.
 
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Old 10-23-21, 01:56 PM
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Welcome to the forums.

The inducer motor spins up as normal and some time in the cycle and I get voltage on the gas valve for a number of seconds, then things spin back down and it tries to start the cycle again.
If you are 100% sure of this..... your igniter is not working. It's either bad or not getting power from the board. You can turn off the safety switch. Turn it back on and observe the cycle.

The draft inducer runs first, the pressure switch proves, then the igniter heats and then the gas valve opens.
 
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Old 10-24-21, 04:10 PM
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I went ahead and did more troubleshooting on it today, looking at the igniter as you said I should, and I found that I get absolutely no voltage sent to the igniter. I did further testing on the pressure switch and it is good.

I shut off the gas, reset the unit, pulled the cover off over where the igniter and heat exchange is, and did the startup sequence and got no real voltage other than 0.9 VAC when ti tried to work, then 0.5VAC when it wasn't cycling up.

I did a bit more investigation and marked/took pics of the wiring, then pulled them, and then pulled the board out, and all I have to say is this sucks. I wonder how much one of these boards costs.



 
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Old 10-25-21, 12:01 AM
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Make sure you use the correct model number when ordering a board.
TUN2B080A9V3VBA is not correct.

Possibly TUH2B080A9V3VBA

I'm seeing CNT07991 as the control board and yes it's very expensive...... $500.
 
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Old 10-25-21, 04:37 AM
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TUH2B080A9V3VBA is the right one. I had to take a pic of it at a weird angle and wasn't sure if that was an N or an H, but zooming in it is an H, so thank you for that clarification and having me do a double check on it. The first part is kind of blurry.

I am hoping one of the local places has one. There are two of us up here in my area for Honeywell and we work together all the time. he does HVAC so might have a bit of a discount like I do when it comes to security, access control, network, computer hardware, and CCTV stuff.
 
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Old 10-25-21, 12:30 PM
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New board in and I have heat! I used some of my good sealer to make sure that there are no leaks that can get to the board.
 
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Old 10-25-21, 03:47 PM
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Good job. Thanks for letting us know how you made out.
 
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