I serviced a Mitsubishi MXZ-3C30NAHZ2 multi-zone heat pump that was experiencing poor cooling performance. Diagnostics revealed low refrigerant levels and a malfunctioning expansion valve. I located and repaired a small refrigerant leak at a flare connection using Nylog Blue thread sealant, then recharged the system with R-410A to the correct levels. I also replaced the faulty expansion valve. After these repairs, I recalibrated the system and confirmed proper operation in both cooling and heating modes.