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A short explanation, on what this does, Very simple it is a flame sensor, so it feels the flame. But it only has 1 to 2 seconds to feel the heat from the flame, and send the signal to the control board. any delay and the board shuts off the gas, and the furnace tries again. So that little discoloring on the flame sensor acts as insulation preventing the flame sensor from feeling the heat, in the very short time allowed. In short, it needs to be cleaned.

Clean flame sensor