The brief
Kelson drove his Golf in complaining that the aircon was leaving him sweating, no cold air, and after testing it turned out to be the air conditioning compressor. The compressor is the heart of the aircon, the pump that compresses the refrigerant and pushes it round the system to do the cooling. The compressor works hard every time the aircon is on, and over the years the internals wear, the clutch can fail, or it can seize. When it goes, the refrigerant doesn't get pumped, so there's no cooling no matter how full the system is. A failed compressor doesn't recover, so it gets replaced, and because debris from a worn compressor can spread through the system, the lines are flushed, the receiver-drier renewed, and the system properly recharged.
The diagnosis
Extensive tests and investigation narrowed the cooling loss to the air conditioning compressor, which had failed, which is why there was no cold air. The condenser, the evaporator and the rest of the system were sound otherwise, just needing flushing of any debris. That's a compressor replacement with the system flushed, a fresh receiver-drier and a full recharge, rather than a top-up that wouldn't help a dead compressor.
The work
The refrigerant was recovered, the failed air conditioning compressor removed and a new genuine VW-spec compressor fitted, the system lines flushed of any debris, and a new receiver-drier installed. The system was pulled down to a long, deep vacuum to confirm it held and to dry it out, then recharged with the correct weight of refrigerant and the right oil charge. The drive belt was checked. A check at the vents confirmed cold air at idle, holding cold under load, with no leak, all turned around in a day.
The outcome
Cold air at the vents at idle and on the move, strong cooling, a fresh drier, the charge holding with no leak, and the car back within the day. Kelson got the Golf back with the aircon working properly again. A failed compressor needs replacing, not topping up, so doing it with a flush, a new drier and a proper recharge fixed the cooling for good.