What is the most likely cause if the outdoor coil is frozen solid during a "no heat" service call?

When dealing with a frozen outdoor coil during a "no heat" service call, the most likely cause is that the outdoor fan is inoperative. The outdoor coil relies on the fan to blow air across it to facilitate the heat exchange process. If the fan is not functioning, air circulation is severely reduced or completely halted, which prevents the coil from effectively shedding heat. This leads to the refrigerant not being able to absorb enough heat from the outdoor air, causing it to freeze and form ice on the coil.

The situation can often be exacerbated by the ambient outdoor conditions, particularly if temperatures drop near or below freezing. Without the fan functioning, ice can quickly accumulate, leading to a solid freeze on the coil. Checking the operational status of the outdoor fan is therefore critical when diagnosing this issue.

While other factors, such as low refrigerant or a faulty coil, could potentially lead to freezing, the immediate lack of airflow caused by the inoperative outdoor fan is the most direct reason for the situation described. A broken thermostat usually would not produce this specific symptom of a frozen outdoor coil during a heating cycle, as it more often contributes to control issues rather than physical freezing.