The brief
A BMW came in that needed topping up with refrigerant far too often, and when you're doing that, the odds are the cooling coils are failing and a leak has started. The aircon is a sealed system, and refrigerant only leaves it through a leak, so a system that keeps going low has one somewhere. The cooling coil, the evaporator, sits behind the dashboard and is the part that actually chills the air on its way to the vents. When it deteriorates and springs a leak, it should be replaced straight away, because topping up just sends fresh gas out the same hole. The catch is that getting at it means dismantling the dashboard, which takes skill, unskilled hands leave the dash misaligned and squeaking afterwards, so it's a job for the aircon team to do right.
The diagnosis
The leak check traced the lost refrigerant to the cooling coils behind the dash, deteriorated and leaking, which is why the system kept going low. The compressor, the condenser and the rest of the system were sound otherwise. That's a cooling coil replacement, which means the dash coming out carefully, plus a full recharge, rather than a top-up that would leak straight back out.
The work
The refrigerant was recovered, the dashboard dismantled carefully to reach the heater box, and the deteriorated cooling coil removed and a new genuine BMW-spec coil fitted. The system was pulled down to a long, deep vacuum to confirm it held and to dry it out, then recharged with the correct weight of refrigerant and the right oil charge. The dash was reassembled with care, every part back in its place, so there are no rattles or squeaks afterwards. A check at the vents confirmed cold air at idle, holding cold under load, with no leak, and the dash back together solid and quiet.
The outcome
Cold air at the vents at idle and on the move, strong cooling, the charge holding with no leak, and the dash back together solid, with no squeaks or rattles. The car went home with the aircon working properly again. A leaking cooling coil only loses more refrigerant the longer it runs, so replacing it and recharging it right fixed the cooling for good, and putting the dash back the right way kept it rattle-free, done right the first time.