Flame rectification is an electrical process that causes a low level DC current to be sent from the flame rod through the flame, and back to ground. The ignition controller is programed to look for this DC current and make a “green light or red light” decision based on the strength of this current.
Some of the strengths of this system are a fast response (within microseconds) and it is impossible to bypass or prevent. One of the weaknesses is that the amount of flame current is very low and can be diminished fairly easily.
These working components can become dirty or corroded over time. Once the electrical path is interrupted, the flame current can be diminished even though the flame has been successfully established and everything is operating correctly.