Which of the following conditions would denote a properly functioning thermal expansion valve?

A properly functioning thermal expansion valve is indicated by a consistent refrigerant temperature at the outlet. This consistency signifies that the valve is accurately regulating the flow of refrigerant into the evaporator coil, which is crucial for efficient cooling. When the thermal expansion valve operates correctly, it maintains the desired superheat level, ensuring that only vapor enters the compressor, preventing potential damage and optimizing the system's performance.

Liquid refrigerant in the sight glass might suggest improper functioning or issues elsewhere, as a correctly functioning system typically shows vapor. High superheat readings can also indicate potential problems, such as insufficient refrigerant or restriction in the system, which would not be ideal for a well-functioning valve. Frost buildup on the valve is another indication of malfunction, as excessive condensation or frost implies that the valve is either stuck open or there is an issue with the refrigerant flow, which can lead to inefficiencies and damage. Overall, consistent refrigerant temperature at the outlet reflects the effective control and balance that the thermal expansion valve should maintain within the system.