The brief
The 530i had needed its aircon gas topped up twice within three months, and each time the cooling had gone weak and warm again as the charge bled away. He brought it in to find out where it was going. Refrigerant that keeps escaping points at a leak in the AC, and on these cars the cooling coil, the evaporator buried behind the dash, is a common source, along with the lines that feed it. Refrigerant runs through the coil, cabin air blows over it and comes out cold, and the moisture that condenses on it drains away outside. When the coil pinholes it can't hold a charge, which is the weak, warm air. Topping it up just buys a few weeks, so it needs the leak found and fixed, not another recharge.
The diagnosis
A pressure check showed the system undercharged, and a leak test traced it to the cooling coil behind the dash, with the lines, the compressor and the condenser all checking clean. That's a coil replacement. A leaking evaporator only leaks more, so the coil had to come out, which on this car means going behind the dash.
The work
The remaining refrigerant was recovered, the dash dropped back far enough to open the HVAC box, and the failed cooling coil lifted out. The housing was cleaned, and a new genuine BMW-spec coil went in with fresh seals. Then the dash was rebuilt, the system pulled down to a long, deep vacuum, and recharged with the correct weight of refrigerant. A check at the vents confirmed cold air at idle and the charge holding.
The outcome
Cold air at the vents at idle, strong airflow, and the refrigerant charge holding steady. The 530i went home with the aircon working the way it should. A leaking evaporator is a big job because of where it sits, but it's a once-and-done one, so changing it and recharging properly fixed the cooling rather than chasing the charge round again.