The flame sensor should be cleaned annually with a light sand paper to remove any oxidation on it. The flame sensor works to detect the flame by a small current being sent down it. That current actually travels through the flame to the burner. This creates a very small short, and the control board recognizes a flame is present. It does not work by just the flame rod being in the flame and getting hot. This is why it is very important that the flame rod remain clean. If the ceramic on the flame sensor is cracked, it will need to be replaced. If there are two rods because your have spark pilot assembly, lightly sand both rods.