The brief
The A4's aircon had stopped doing its job. Warm air at the vents instead of cold, even with the temperature wound all the way down.
Two other signs came with it. A squealing noise from the front of the engine bay whenever the AC compressor engaged. And refrigerant pressure was dropping over time, which is something the system tells you on the scan tool.
In Singapore, weak aircon is not a wait-and-see fault. It is one of the things you actually want fixed today, not next week.
Three signs together pointed at the compressor itself, not just a low refrigerant top-up. A top-up would give you a few weeks of cold air and then the same problem all over again, which is the worst kind of repair: a bill, and the same fault still there.
The diagnosis
We started with a pressure check on the system. It came back low on refrigerant, which on its own only tells you there is a leak or a failure somewhere. It does not tell you where.
Next we listened at the compressor with the AC clutch engaging. The squeal tracked with the bearings inside the compressor pulling in under load. That is the kind of sound a compressor makes when it is grinding its internal parts together.
A compressor that has been grinding internally is also a compressor that has been shedding metal debris into the AC lines. Replacement is half the job. The other half is flushing the whole loop, because if you leave the metal debris in there, it ends up inside the new compressor and you are doing the same job twice in a month.
So the call was full compressor replacement, plus a system flush, plus a new receiver-drier (which is the filter that catches any remaining debris).
The work
Refrigerant recovered first into the reclaim machine. You cannot vent it to atmosphere, both because it is bad for the environment and because the regulators have rules.
Failed compressor off next. Then the flush. We ran the cleaning solvent through the AC lines and the condenser to chase out every trace of metal debris from the failed compressor. That part takes time and patience to get right, but skipping it costs you the next compressor.
In went a new Audi-spec compressor and a fresh receiver-drier. Both come with the right amount of refrigerant oil pre-charged, which is one of those details that often gets missed and quietly kills compressors early.
Then the system gets vacuumed down to a deep pull, which boils off any moisture that has snuck in. Once the vacuum holds steady for the prescribed time, you know the whole loop is sealed and dry.
Refrigerant gets charged in by weight, not by guess. The factory spec is the factory spec.
The outcome
Cold air at the vents the moment the system was switched on at idle. No squeal from the front when the AC engaged. The system holding pressure cleanly across idle and load.
We also confirmed cabin temperature came down properly with the windows up in the workshop, which is the realistic test for a car driven in Singapore.
The A4 went home with the aircon working the way it should. For the owner, the practical win is straightforward: a cold cabin within a minute of starting the car, which is the difference between a comfortable drive and a sweaty one in this climate.
And by doing the flush and the receiver-drier as part of the job, the new compressor should get its full service life.