What should be done immediately after closing the suction shut-off to add oil to a system?

The correct course of action after closing the suction shut-off to add oil to a system is to stop the compressor and isolate it by closing the discharge valve. This step is crucial because it ensures the system is safely depressurized and prevents any refrigerant from circulating while the oil is being added. Isolating the compressor protects both the technician and the equipment, as operating the compressor while adding oil can lead to potential system damage or unsafe conditions.

Stopping the compressor is essential to ensure that no high-pressure refrigerant is in the system when the oil is introduced. Closing the discharge valve further secures the system by preventing backflow of refrigerant, allowing the technician to add oil without compromising the system's integrity or safety.

In contrast, other options do not address the safety and integrity of the system under the circumstances. For example, opening the liquid line valve before ensuring the system is safe could lead to refrigerant leaks or other hazards. Reducing the temperature of the system may not be relevant and does not directly relate to the oil addition process. Running the system for a short duration before adding oil can also risk oil starvation to the compressor, which can damage the unit. Thus, isolating the system is the most prudent and correct action.