The brief
The A4 had aircon that had gone weak and warm, and at over 150,000 km the owner had topped up the refrigerant only to watch it disappear again. He brought it in. Refrigerant that keeps escaping on a high-mileage car often points at the condenser. The condenser sits right at the front of the car, ahead of the radiator, and its job is to shed the heat the aircon has pulled out of the cabin so the refrigerant can cycle round and do it again. Years of road grit and moisture corrode its thin aluminium tubes and fins, and eventually one pinholes and the refrigerant leaks out. With the charge low the aircon can't cool, which is the warm air, and a corroded condenser only gets worse from there.
The diagnosis
A pressure check showed the system low on refrigerant, and a leak test traced it to the condenser, which had corroded through and was weeping at the front of the car. The lines, the compressor and the evaporator all checked clean. That's a condenser replacement, not another recharge. A pinholed condenser only leaks more, and topping it up just buys a few weeks, so the condenser had to come out.
The work
The remaining refrigerant was recovered, the front end opened up enough to reach the condenser, and the corroded unit removed. A new genuine Audi-spec condenser went in with a fresh receiver-drier and new O-rings, 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 at idle and the charge holding.
The outcome
Cold air at the vents at idle, strong cooling, and the refrigerant charge holding steady. The A4 went home with the aircon working the way it should. A corroded condenser is a leak that won't stop on its own, so changing it and recharging properly fixed the cooling rather than chasing the charge round again.