The brief
The Golf had the aircon blowing warm with a musty smell in the cabin, and the owner had found a damp patch in the front passenger footwell. Three signs pointing at the cooling coil and its housing.
The cooling coil, the evaporator, is the part of the aircon buried in the dashboard. Refrigerant runs through it, the cabin air blows over it and comes out cold, and the moisture that condenses on it is supposed to drain away outside. A leaking coil can't hold a refrigerant charge, which is the warm air, biofilm on a tired coil gives the musty smell, and a degraded drain seal lets the condensate run into the footwell instead of out the bottom. All of it traces back to that one part.
The diagnosis
A pressure check showed the AC system low on refrigerant with no visible leak under the bonnet. A leak test confirmed the cooling coil as the source, the lines, compressor and condenser all checked clean. And the damp footwell traced to the condensate routing wrong past a degraded drain seal.
That's a coil replacement, which means going behind the dash. And since the housing is open once you're in there, it gets cleaned of the biofilm before the new coil goes back, so the smell goes with the old coil.
The work
The remaining refrigerant was recovered, the dash trim came out to reach the evaporator housing, and the failed cooling coil was lifted out. The housing was cleaned of the biofilm causing the musty smell, and a new VAG-spec coil went in with fresh seals, including a fresh drain seal so the condensate routes out properly.
Then the dash and trim were rebuilt, the system vacuumed to a long, deep pull, and recharged with the correct weight of refrigerant.
A check at the vents confirmed cold air and a dry footwell.
The outcome
Cold air at the vents at idle, no musty smell, and the footwell dry.
The Golf went home with the aircon working the way it should. A leaking evaporator is a big job because of where it lives, but it's a once-and-done one, and doing the housing clean and the drain seal while it was open means the smell and the wet footwell are sorted at the same time, not just the cold air.