The brief
The Golf's aircon had lost its bite even on the maximum cooling setting, a musty smell came on whenever the AC ran, and the passenger footwell carpet had started showing a damp patch. Three symptoms all pointing at the evaporator core, the part of the aircon buried behind the dash.
Refrigerant runs through the evaporator, cabin air blows over it and comes out cold, and the moisture that condenses on it is meant to drain away outside the car. When the core leaks, it can't hold a refrigerant charge, which is the weak cold air. Damp sitting on a tired core grows the biofilm that gives the musty smell. And when the drain seal next to it has degraded, the condensate runs into the footwell instead of out the bottom. All three trace back to that one part.
The diagnosis
A pressure check showed the system undercharged, so refrigerant was escaping somewhere. The drain line itself was clear, which meant the wet footwell was condensate being routed past a degraded gasket rather than a blocked drain. Behind the dash, the evaporator core was the source of both the refrigerant leak and the wet path.
So this was a core replacement, not a recharge. A leaking evaporator only leaks more, and topping it up just buys a few weeks, so the core 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 evaporator core lifted out. A new VAG-spec core went in with fresh seals and a new expansion valve, the drain seal renewed so the condensate routes out properly.
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 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 sits, but it's a once-and-done one, and doing the seals and the expansion valve while it was open means the cold, the smell and the wet footwell are all sorted in the same visit.
