The brief
The A4's aircon had been losing its bite at idle, the cabin caught a musty smell every time the AC kicked on, and a wet patch had been growing on the passenger footwell carpet.
Three symptoms that all point to the same place: the evaporator core, the cooling coil behind the dash. Weak cold air says the system has lost refrigerant. The musty smell is biofilm building up on a coil that is not draining properly. And a wet carpet on the passenger side is condensate ending up where it should not, which it does when the seal around the core has degraded. None of those is something a cabin filter swap can reach.
The diagnosis
A pressure check on the AC system showed it undercharged, refrigerant had been escaping. The condensate drain line was clear, which is the easy thing to rule out first on a wet-carpet complaint. But the wet carpet was still there, which meant the condensate was being routed past a degraded gasket on the evaporator core itself.
Behind the dash, the core was the source of both problems, the refrigerant leak and the misrouted condensate. A recharge would have lasted a few weeks before the leak emptied it again, and it would have done nothing for the smell or the wet carpet. The core had to come out, which on this car is a full dashboard-out job.
The work
Recovered the remaining refrigerant first. Then dropped the dash, the whole assembly, to get at the HVAC box, because the evaporator lives deep behind it and there is no shortcut.
Removed the failed evaporator core, fitted a new Audi-spec replacement with fresh seals and a new expansion valve, since the expansion valve sits right there and is not something you refit once you have it out.
Vacuumed the system down to a deep pull, recharged with the correct refrigerant volume and oil charge, reassembled the dash, and fitted a fresh cabin filter to seal off any trace of the old smell.
The outcome
Cold air at the vents at idle. No musty smell. The footwell dry.
The A4 went home with the AC working the way it should, and the smell gone at its source rather than masked. For the owner, that means a cabin that cools properly and smells clean, and a problem genuinely fixed.
An evaporator job is a big one, but it is the right one when the core is the fault, and doing the expansion valve and a fresh cabin filter as part of it means the whole system is set up to stay right for the long run.