What does it indicate if an ohmmeter reads infinity between the high speed and common wires on a 120 VAC furnace fan motor?

When an ohmmeter reads infinity between the high speed and common wires on a 120 VAC furnace fan motor, it indicates that there is an open circuit in the motor windings. In this context, "infinity" means that the resistance is so high that current cannot flow, which is typically a sign that the electrical path has been interrupted. This often means that the winding is broken or there is a disconnection somewhere in the motor, preventing it from operating properly.

On the other hand, if the motor were functioning well, you would expect to see a finite resistance reading indicative of intact windings, typically a low ohmic value. A shorted winding would result in a very low resistance reading rather than infinity, indicating an undesired connection within the winding. Cleaning the fan may help with airflow or performance issues but would not directly affect the measurement of resistance between the windings.