What should I do if my Ruud Ultra Plug-in Heat Pump with HydroBoost (120V Shared Circuit) indicates Alarm Code A105?

CAUTION: For your safety, DO NOT attempt repair of electrical wiring, controls, heating elements, or other safety devices. Refer repairs to qualified service personnel.

Possible Cause: Heating source defaults to elements due to heat pump temperature malfunction. What To Do:

1. Carefully inspect and ensure the connections of the wiring and sensors are secure.
2. If you possess a multimeter, capture the Ohm reading on the thermistors. For accurate interpretation, utilize the Ohm chart to align readings with the prevailing ambient temperature.
3. Using the device’s interface, move to Service –> Sensors. A display of -40°F signifies the circuit is open, while a +250°F reading indicates a shorted circuit. For accuracy, deactivate the unit, detach the thermistor from its board, and re-assess the Ohm reading.

This answer is applicable for the following model: Ruud Ultra Plug-in Heat Pump with HydroBoost (120V Shared Circuit).