The brief
The A4's aircon had been losing its bite, and the cabin had been catching a musty smell. The owner had already tried swapping the cabin filter, which is the right first move for a musty smell, but it had not fixed it.
He brought it in to find the actual cause. A musty smell that a fresh cabin filter does not cure usually means the problem is deeper in the system, behind the dash, at the evaporator core itself, where damp and biofilm build up if condensate is not draining the way it should. And a fresh filter cannot reach that.
The diagnosis
A pressure check showed the AC system undercharged, so it was losing refrigerant somewhere. The condensate drain line was clear, which on a damp-smell complaint is the easy thing to rule out first. But the passenger footwell carpet was damp on inspection, which tells a different story: condensate was being routed past a degraded gasket on the evaporator core, ending up where it should not.
Behind the dash, the core itself was the source, both of the refrigerant leak and of the misrouted condensate. A new cabin filter cannot fix that. The evaporator 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. There is no shortcut on this one; the evaporator lives deep behind the dash and the dash has to come out.
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 the kind of part you do not refit once you have it out.
Vacuumed the system down to a deep pull, recharged with the correct refrigerant volume, reassembled the dash, and fitted yet another 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 traced to its actual source rather than masked. For the owner, that means a cabin that cools properly and smells clean, and a problem genuinely fixed rather than chased with another filter.
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 filter as part of it means the whole system is set up to stay right for the long run.