The brief
The A4's aircon had been losing performance gradually. The owner had topped it up a few months back, but the cold was already fading again. And a small wet patch had been forming under the front of the car.
A top-up that does not last, plus a wet patch right at the front: that is refrigerant escaping from the front of the AC system, where the condenser sits. A recharge buys a few months because it replaces the lost refrigerant, but if there is a leak, the new gas goes the same way as the old. The leak needs finding.
The diagnosis
A pressure check showed the AC system undercharged again, confirming the loss was ongoing. A visual inspection of the condenser at the front of the engine bay turned up the source: a small corrosion pinhole at one of the tube ends.
That is a classic age failure for a condenser that has spent years behind the front grille in Singapore's humidity, with road grit and moisture working at the thin aluminium tubes. There is no repair worth doing on a corroded condenser tube. The compressor and the rest of the loop checked clean, so the condenser was the only failed part. Replacement, not just a recharge.
The work
Recovered the remaining refrigerant into the reclaim machine first. Then removed enough of the front bumper to get at the condenser, which on this car means a fair amount of front-end strip-down since the condenser sits ahead of the radiator.
Fitted a new Audi-spec condenser plus a fresh receiver-drier. The drier always gets replaced when the system is opened up, because it absorbs moisture and once it has been exposed to air it has done its job.
Vacuumed the loop down to a deep pull to boil off any moisture, recharged with the correct refrigerant volume and oil charge by weight, then pressure-tested before the car went out.
The outcome
Cold air fully back, at idle and on the move. No wet patch under the front of the car. The system holding pressure.
The A4 went home with the AC working the way it should. For the owner, in this climate, that is the difference between a cabin that cools properly and one that fades on you in traffic.
And by doing the receiver-drier and a proper vacuum-and-recharge as part of the job, the system is set up to hold its charge this time, rather than being back for another top-up in a few months.