The brief
Mr Tan brought his A3 in with the check engine light on and a rough idle, and the diagnosis pointed at the oil separator in the crankcase ventilation system. The oil separator, part of the crankcase ventilation system, sometimes called the PCV, separates the oil mist and contaminants from the crankcase gases and recirculates the cleaned air back into the intake to be burned, holding onto the oil. When it fails, raw oil mist gets pulled straight into the intake, which burns oil and can foul things, and the failed unit upsets the crankcase pressure, which leans out the idle and trips the light. A failed oil separator doesn't recover, so it needs replacing.
The diagnosis
Diagnostics confirmed it, fault codes for the crankcase ventilation system and a lean mixture, and a check showed the separator wasn't holding pressure as it should, pulling oil through the breather. The intake, the injectors and the rest of the engine checked out. That's a replacement. The separator and its diaphragm are sealed into the breather unit, you don't rebuild it on the car, so the call was a complete unit.
The work
The old oil separator was removed and a new genuine Audi-spec unit fitted with fresh seals, the breather hoses checked over while everything was apart. Then the fault codes were cleared and the engine's fuelling adaptations reset so it could relearn against a breather system that was working. A road test confirmed a steady idle, the light staying off, and clean response.
The outcome
Steady idle, clean response, no warning light, and the oil consumption back to normal. The A3 went home running properly again. A failed oil separator quietly feeds the engine its own oil and keeps the idle unhappy the longer it's left, so changing it and letting the engine relearn put the running right.