Ruud 90 plus ignition problem
You would probably be money ahead to pull and clean the burners. As far as the low glow on the ignitor, there is a good chance the ignitor itself has a fracture in it and is not glowing completely. If you are getting a blow back that could be an indication of a sooted heat exchanger. This is why I say to pull and clean the burners. This will allow you to see into the heat exchanger. If you see a bunch of black stuff in front of the burners or inside the heat exchanger opening. Call in the pro's for this one, the heat exchanger will have to be pulled out of the furnace and tore apart to be properly cleaned. Hope this helps you understand, let us know if you need further assistance.
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