If the draft pressure switch on a mid-efficiency furnace prevents the ignition system from energizing, what could be a possible issue?

When a draft pressure switch on a mid-efficiency furnace prevents the ignition system from activating, it is typically indicating that there is an issue with the airflow necessary for combustion. A defective induced draft motor is a likely cause because this motor is responsible for creating the negative pressure needed to pull flue gases out of the combustion chamber and ensure that fresh air is supplied for combustion. If the induced draft motor fails, it cannot create the required draft, leading the pressure switch to remain open and inhibit the ignition process.

In this situation, the furnace's safety mechanisms are functioning correctly by preventing ignition when insufficient airflow is detected, which protects against the risk of backdrafting or unsafe combustion conditions. Hence, the failure of the induced draft motor directly relates to the draft pressure switch's functionality and its role in the ignition sequence.