What might be suspected if a technician lights a spark-type ignition furnace, but the spark igniter continues to spark after the main burners light?

When a spark igniter continues to spark after the main burners light, it typically indicates an issue with the ignition control system. In this case, the ignition control board is responsible for managing the ignition sequence, including the spark igniter's operation. If the main burners are already lit, the ignition board should signal the spark igniter to stop sparking. Therefore, if sparking continues, it suggests that the ignition board may not be recognizing that the burners are lit, which points to a malfunction within the board itself.

In contrast, the other options suggest different components or systems that do not directly relate to the mismanagement of the spark igniter's function. A faulty fan switch would not typically cause the igniter to continue sparking; it relates more to airflow and safety cut-off operations. A faulty pilot bi-metal is more associated with a pilot assembly and the extinguishing of a pilot flame, which is not applicable in this scenario since we are dealing with main burners being lit. Thus, the ignition board is the most logical source of the problem, making it the correct answer.