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Experience:  15yrs in the HVAC industry with certificates and degrees.
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Model p9ucd12no76o1a Every time the blower motor starts the

Customer Question

Model p9ucd12no76o1a
Every time the blower motor starts the Gas shuts off and the red continuous light stays on.
The gas will not ignite again. unless I turn thermostat down then up. or turn off power and then turn it on again. Then the cycle starts again. Purge is good, igniter comes on, flame comes on, blower motor starts, then flame off. about 3 or 4 minutes/cycle.
I change the fan/limit switch but still doing same thing. I am not sure what is the problem.
Do you have any suggestions please.
Submitted: 5 months ago.
Category: HVAC
Expert:  HVAC Guru replied 5 months ago.
Hello and thanks for using this service, if you have been waiting do not worry or get upset. I am HVAC Guru an extremely experienced tech on the site and I try to get to as many customers as I can. I just picked up your question now. NOTE it takes time for me to type research and add links so please wait for me to respond back I can only do this so fast. Secondly I am not paid anything until you rate my service good using the STAR RATING LINK at the top of your page, the money is paid to the site not me directly. Answer: If it lights up then goes out right away then repeats this process you need to remove and clean the flame sensor rod off on the unit. The flame sensor rod is what detects the flame after lighting and tells the board to keep sending power to the gas valve to keep the flame going. Over time these get corroded and even go bad failing to send the correct Milliamp signal to the board. Normally a good cleaning with some sand paper will clean them right up and they will work fine again and in other instances they need to be replaced. The part is on the burner manifold and typically will only have 1 wire attached to it and will either be a straight metal rod with a porcelain base or a bent metal rod with a porcelain base. Get that out and clean it off then try it and see what happens. Here is an image of what you would be looking for to take out and clean off with sand paper. CLICK HERE All I ask is please use the Star Rating Link at the top of your page before leaving, that is the only way I am paid anything through the are greatly appreciated so thank you very much for using this service. If you have any other questions just post back to me here anytime!
Customer: replied 5 months ago.
As I said it stay's lite for 3 to 4 minutes until the blower fan is energized by the fan limit control as soon as this happen the fire goes out. Then the red light on the ignition module stays on. Until I either turn off and on the thermostat or the power switch.
then it will go through the cycle again as above.
I have already cleaned the flame sensor 2 days ago.
what do you think is causing the flame to go off everytime at the exact time the fan limit control turns on the blower motor??
I have been fixing furnaces for over 40 years but this has me confused. Please think about it and see if you have a suggestion.
Customer: replied 5 months ago.
I forgot to mention that the thermostat continues to call for heat, and all the limit switches are closed but it will not ignite unless I turn off and on the thermostat or the power switch to the furnace.
Customer: replied 5 months ago.
could it be possible that when the fan limit switch activates the blower motor that this power surge causes the gas valve to turn off? ? Maybe the gas valve needs to be replaced. What do you think? Please help me.
Expert:  HVAC Guru replied 5 months ago.
It sounds like an ignition board issue, you are just resetting the ignition control board when turning it off and back on so I would say that would make sense.
Customer: replied 5 months ago.
well I will try a new one however I just changed it yesterday and it was acting the same as the old one. So I have my doubts about that being the problem. When I go there on Tuesday I was going to try leaving the blower fan on full time and then turning up the thermostat so that there would not be a power surge when the fan limit control turns on the blower fan. Because it will already be running. Just to see if the flame stays on till the thermostat is satisfied. (As it Should) What do you think about this idea?? Also what did you think about a weak gas valve? Thank you for your answers and trying to help. I am sure confused. What the troubleshooting guide said that came with the new ignition control board, was if the system runs until call for heat ends and then it says NO option, Check limit switches. Check senor lead and ground for continuity. Check insulator of the flame sensor or ignitor for excessive temperature. Temperature above 1000 degrees F can ground flame sensor.
In answer to those, the limit switches are all closed, sensor is lead and ground have continuity. But I don't know how to check for excessive temperature. Can you tell me how to do that??
Expert:  HVAC Guru replied 5 months ago.
right when the blower kicks on the unit stops, can you turn the BLOWER to FAN ON and then turn the heat on and see if that works?
Customer: replied 5 months ago.
It takes me 2 hours to drive to this job. Please give me more info. other things to try if turning blower fan on doesn't work.
What to try if the blower fan idea does work. How about my question about how to check the temperature to see if it is excessive ie over 1000F. The job site where the furnace is, is so far away I would like to take extra parts to save time and money. so any ideas you have please let me know. I have already spent 10 hours trying to fix this furnace and any ideas you have would be greatly appreciated. Thank You
Customer: replied 5 months ago.
after reading your last answer can you please clarify for me. You said " so right when the blower kicks in the Unit stops."
The unit does not stop only the flame goes out.
Is this supposed to happen? I did not think so.
What are the possible causes of this ( the flame going out when the blower starts.)??
Customer: replied 5 months ago.
Sorry to ask so many questions but I really want to fix this furnace for the lady that owns it. And I really would like to do it with just on more trip. Thanks again
Expert:  HVAC Guru replied 5 months ago.
Have you waited to see if the flame comes back on?
Customer: replied 5 months ago.
I have waited. The flame does not come back on. I waited for over 1 hour. The only way to get the flame to ignite again is to either turn of and on the thermostat or the main power to the furnace. Then the flame will only stay on till the fan limit control turns on the blower motor. This is 3 to 4 minutes.
Just so you know I am going to buy more parts Tuesday morning, tomorrow, and then I will drive back there I try whatever else we can think of trying.
Expert:  HVAC Guru replied 5 months ago.
Are you sure the fan limit is wired correctly and was the new ignition board a different one wired differently?
Customer: replied 5 months ago.
The Fan limit control which I have also replaced, was a 4 wire. 2 wire for the fan brown and black. The limit was 2 wires blue and purple. The internal brass jumper that is in the fan limit 8" control I left in the fan limit control. As was the way the one that I removed was connected..
The limit board was easy to change wire over also. When I first tried it, it did not go through the whole cycle it stopped at solid red which in the troubleshooting page of directions says to reverse MV1 and MV2. I made this change reversing MV1 and MV2. And then it worked all the way through the cycle. Of course when the fan limit turned on the blower motor the gas shut off. Which is back to where the original problem is.

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