The brief
The Q3's aircon had gone weak: warm or barely-cool air from the vents, noises from the system, and at times no cold air at all. He brought it in, which makes sense in this climate, a working aircon in Singapore isn't optional. Weak, warm air with noises points at the compressor, and a faulty pressure switch alongside. The compressor is the pump that compresses the refrigerant so the aircon can cool the cabin, and the pressure switch tells the system the refrigerant pressure so it knows when to run the compressor. When the compressor wears, the cooling drops off and it gets noisy, and a broken-up compressor can leak debris into the system; a faulty pressure switch can stop the compressor cutting in properly. Between the two, no proper cooling, so the compressor and the switch needed replacing, and the system flushed and recharged with a fresh receiver-drier.
The diagnosis
A check confirmed it, the compressor wasn't building proper pressure, its internals worn and noisy, the pressure switch reading unreliably, and a little debris in the lines. The condenser, the lines and the evaporator checked out, but with a failed compressor the system gets flushed regardless before the new one goes on. That's a compressor and pressure switch replacement, with a fresh receiver-drier and a flush, not something a recharge alone touches.
The work
The refrigerant was recovered, the drive belt released, and the failed compressor unbolted and removed along with the faulty pressure switch. The system was flushed to clear any debris, a new genuine Audi-spec compressor fitted with a new pressure switch, a fresh receiver-drier and new O-rings, the belt set back up, the system pulled down to a long, deep vacuum, and recharged with the correct weight of refrigerant and the right oil charge. A check at the vents confirmed cold air at idle, holding cold under load, and no noise from the compressor.
The outcome
Cold air at the vents at idle and on the move, strong cooling, no noise from the aircon, and the charge holding. The Q3 went home with the aircon working the way it should. A failed compressor only gets worse and can take the rest of the system down with it, so changing it and the switch, flushing the system and recharging properly fixed the cooling and protected the rest of the aircon.