When troubleshooting a "no heat/no cooling" issue in a split system heat pump, what does the operation of the indoor fan verify when the thermostat is turned to the fan on/cool position?

When the indoor fan operates upon setting the thermostat to the fan on/cool position, it verifies that the transformer is functioning properly. The transformer plays a crucial role in stepping down the voltage to the control circuit of the HVAC system, enabling components such as the thermostat and indoor fan to receive the correct voltage supply. If the indoor fan runs, it indicates that the transformer is providing power to the control circuit, allowing for the operation of the fan.

This functionality helps rule out some issues in the system, as it demonstrates that there is power and that the basic control components are operational. If the fan is working but there is no cooling or heat, it points towards other potential issues, such as compressor failure or refrigerant problems, rather than indicating a problem with the transformer itself.