How does the ignition module differentiate between a flame and a sensor shorted to ground?

The ignition module differentiates between a flame and a sensor shorted to ground primarily by detecting DC current. When a flame is present, the flame sensor generates a small amount of DC current as a result of the ionization of the flame; this is a fundamental characteristic of how flame sensors operate. The presence of flame allows the DC current to flow, which the module recognizes as a normal operational state.

In contrast, if the flame sensor is shorted to ground, the expected DC current flow is interrupted or altered, signaling to the ignition module that there is a fault instead of a flame. This ability to sense the specific nature of the current—whether it is the expected ionization current from a flame or an abnormal state such as a short circuit—enables the module to effectively identify the operational status of the burner system and make appropriate adjustments or shut down as necessary for safety.

While options related to measuring ignition time, sensing AC current, or analyzing flame color may play roles in the ignition and monitoring processes, they do not directly pertain to the fundamental method of how the module interprets the difference between flame detection and a sensor fault condition.