Hello HVAC1000,
Thank you for the information. My Trane X80 furnace is 13 years old maintained in a good condition. But recently when room thermostat closes, the furnace starts it's normal cycle (fan and igniter) but skips opening the gas valve, then the igniter goes off and the fan continues then after a while the other fan starts to blow cold air to the rooms. Some times, maybe two out of 10 it works without any problem. I have to mention that after cleaning the flame detector, it works only for few cycles without problem and then it shows the same behavior.
I inspected the control board carefully and found no sign of burning or discoloration or smell. I wonder if the problem is electrical or mechanical. I would appreciate your advice and opinion if possible.

Thanks,
Benam

Try to revisit cleaning the flame sensor again. Fine sand paper or steel wool works well.

While cleaning check the condition of the wire or cable that connects the flame sensor to the board. Many times if you clip a bit of wire off both ends and reattach it will also make sure the connections are good and tight.

Note: The flame sensor and wire does not generate much current and if the connections are not perfect much is lost in translation if you know what I mean.

Next attach a volt meter to the wires going to the gas valve. During the furnace startup procedure 24 volts should show on the meter indicating that the control board is sending a signal for the gas valve to open. If voltage present then the gas valve is probably defective if no voltage then the circuit board is probably defective.

There is a small window to see the voltage at so pay close attention.

Hello HVAC 1000m,
You were absolutely right on cleaning the connectors. Recleaning the flame sensor didn't work. Then I unpluged all connectors and wires from the controller and cleaned the back of the board (which looked
Already clean and shining) and connectors with a dry tooth brush then reconnected wires and connectors and powered the system up and SUCCESS!!

I believe, as you mentioned, the problem was dirty connectors. The system is still running since cleaning and I have my fingers crossed to see what happens next tim it starts with thermostat. I'll keep you posted anyway and thank you again.

jbenam

Good deal and I hope it keeps running.

Quote:

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Originally Posted by hvac1000

Good deal and I hope it keeps running.

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It worked for 16 hours qiute normal but at 5 AM I turned it off because it was blowing cold air. At 10 AM I turned it on and it was good again.
Now it is off by thermostat and I am waiting to see what happens when it starts.:confused:

I hate intermittent stuff. It could be a weak capacitor on the circuit board.

Quote:

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Originally Posted by hvac1000

I hate intermittent stuff. It could be a weak capacitor on the circuit board.

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Since 10AM it has been running normal but If the problem is from a weak capacitor, I have enough electronic experience to change it with a new one provided that I know which capacitor it is or at least have access to the schematic diagram of the controller.

Thaks again for the help.

Quote:

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Originally Posted by jbenam

Since 10AM it has been running normal but If the problem is from a weak capacitor, I have enough electronic experience to change it with a new one provided that I know which capacitor it is or at least have access to the schematic diagram of the controller.

Thaks again for the help.

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I have no schematic of the board since that is considered proprietary info. My guess was capacitor since they change there value quicker than other board components.

It stopped working again, now I'm thinking about buying an equivalent controller 50A55-843 but I'm not sure if the connections are the same or any wiring modifications are needed. Could you please help?
Thanks

What is the exact part number on the old board?

If you are thinking of using the 50A55-843 board here are the directions.

http://www.emersonclimate.com/Docume.../0037-6265.pdf

The model # of the old controller is
50A50-471. Do I need to do any modification in wiring or changing any switch positions on the new cotroller or just plug and play? I'd appreciate your advice.

I suggest you follow the directions that come with the new style board exactly!

I also suggest you identify/label every wire as it is removed from the old board and check to be sure it goes exactly where the directions for the new board tells you to place it.

You will also be using the Trane jumper that is supplied for the new board. Just install it as instructed.

When done recheck all connections.

Ok, many thanks. It'll take some while to receive the controller. So far the system is working intermittently.
I also have suspicion on the flame sensor thermocouple I wonder if there is a simple way to test it's function.

Only way to be sure