If the pressure switch is open and the inducer motor is running but the ignitor does not come on, what is the likely diagnosis?

In this scenario, the pressure switch being open while the inducer motor is running indicates that the system is not able to verify that there is adequate venting for combustion gases. The typical function of the pressure switch is to ensure that there is proper airflow through the flue and that exhaust gases are being properly vented outside. If the pressure switch remains open, it means it hasn't detected the necessary conditions for safe operation, which would prevent the ignition sequence from proceeding.

Given that the inducer motor is running, it implies that the motor is attempting to create negative pressure to draw air into the combustion chamber and push out exhaust gases. However, if there is a blockage or significant restriction in the flue pipe—resulting in a clogged condition—the pressure switch will not close, thereby preventing the ignitor from activating. The condition of the flue pipe directly impacts the operation of the furnace, as a clogged flue would inhibit the necessary airflow.

If other choices were examined, a faulty ignitor would typically manifest itself if the pressure switch were closed but the ignitor failed to heat up. A gas leak would present itself as a critical safety hazard and usually would lead to different symptoms, such as the smell of gas or fault codes indicating a safety lock