If the pressure switch is stuck closed in a furnace, what would most likely occur?

When a pressure switch is stuck in the closed position in a furnace, it typically signifies a malfunction in the safety mechanisms designed to ensure proper airflow and operation of the appliance. The pressure switch is crucial because it indicates whether there is sufficient airflow through the venting system, which is essential for safe combustion.

If the pressure switch remains closed, the furnace may interpret this as an indication that everything is functioning correctly. However, since the pressure switch is unable to detect real-time airflow conditions accurately, this can lead to unsafe situations. To prevent damage or safety hazards, many furnaces are equipped with a safety feature that triggers a lockout mode. In this mode, the furnace will cease operation, allowing for troubleshooting and avoiding potential overheating or combustion risks.

Thus, the most likely occurrence when a pressure switch is stuck closed is that the furnace will lock out to ensure user safety and prevent further complications.