We had Four Seasons come out and look at our furnace on 3/23. The technician's name was Justin and he seemed very knowledgeable and friendly. He quickly discovered that the flame sensor was dirty and cleaned it. He also noticed that we had some spray paint in the room next to our furnace (my wife is into art and uses spray paint in some of her work) and said it was possible the spray paint caused the flame sensor to get dirty. He advised my wife to not spray near the furnace (she only does this during the winter time) to help keep the sensor from getting dirty unnecessarily.
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