The brief
The 520i had lost airflow at the vents, a musty smell came up every time the air-con kicked in, and a wet patch had been growing on the passenger footwell carpet. Three symptoms all pointing at one place, the evaporator behind the dash.
The evaporator is the cold coil the cabin air blows through, the part that actually does the cooling. Refrigerant flashes to gas inside it and pulls heat out of the air. When it leaks, you lose cooling and airflow; the moisture it normally drains away outside the car instead finds its way onto the carpet; and damp left sitting in the HVAC box grows the mould that gives the musty smell.
The diagnosis
A pressure check on the air-con system showed it undercharged, refrigerant had been escaping somewhere. The condensate drain line was clear, so the wet carpet was not a blocked drain. Behind the dash, the evaporator core itself was the source of the leak, and the water on the carpet was condensate that had been finding its way past a degraded seal at the box.
There is no recharging your way out of a leaking evaporator, the gas just escapes again. The fix is a new core, and getting to it means dropping the dashboard, which is the bulk of the labour on this job.
The work
The remaining refrigerant was recovered, the dashboard dropped enough to open the HVAC box, and the failed evaporator core removed. A new BMW-spec core went in with fresh seals and a new expansion valve, then the dash was reassembled, the system vacuumed to pull out any moisture, and recharged with the correct refrigerant volume.
A fresh cabin filter went in while the box was open, and the vent temperature and airflow were checked before the car went back to the owner.
The outcome
Cold air back on the vents, full airflow, no musty smell, and the footwell dry.
The 520i went home with the air-con working the way it should. For the owner, in Singapore's heat, that is not a small thing, and replacing the core properly, with a new expansion valve and a fresh cabin filter, means the system is sealed and clean again rather than limping along on a top-up that would have leaked back out.