What condition prevents the pressure switch in the furnace from closing?

The condition that prevents the pressure switch in the furnace from closing is a plugged vent. The pressure switch in a furnace is designed to ensure that there is adequate airflow through the venting system. It closes once it detects that the proper negative pressure is achieved, indicating that venting is functioning correctly. When the vent is blocked or plugged, the airflow is disrupted, which prevents the pressure switch from reaching the necessary pressure differential to close. This situation ensures that the furnace does not operate, as continued operation under such circumstances could lead to dangerous conditions, such as the accumulation of combustion gases.

In contrast, low gas pressure would typically not directly affect the closure of the pressure switch, as it is primarily concerned with airflow rather than fuel supply. Proper venting is a condition that would allow the pressure switch to close, while a functional draft inducer also contributes positively to the balance and operation of the venting system.