The brief
The A3 had poor acceleration, the response off the pedal feeling lazy, the idle running uneven, and a couple of times it had stalled unexpectedly, with a check engine light on the dash. He brought it in. Those symptoms point at the throttle body. It's the valve that meters air into the engine, opened by a small electric motor that takes its orders from the accelerator pedal. Over time carbon builds up on the valve, the motor and the position sensors inside it wear, and the valve stops tracking its commands cleanly, so the engine gets the wrong amount of air at the wrong moment, which is the lazy response, the rough idle and the stalls, and the computer sees the mismatch and flags it. A clean buys time on a carbon-fouled one, but a worn actuator needs replacing.
The diagnosis
Live data on the scanner showed the throttle butterfly tracking jerkily through its range rather than following the command smoothly, and the codes pointed at the throttle valve control and its position sensors. So the actuator itself was failing, not just dirty. That's a throttle body replacement. You don't rebuild the actuator, so the unit gets changed and the engine taught the new positions.
The work
The intake hose came off, the throttle body was unbolted, and a new genuine Audi-spec unit fitted in its place, the harness reconnected. Then the throttle adaptation routine was run on the scanner so the ECU learned the new closed and full-open positions, and the stored codes cleared. A road test followed to confirm the response was sharp again from a standing start.
The outcome
Steady idle, sharp response from a standing start, no stalling, and the check engine light out after a drive cycle. The A3 went home running cleanly through the rev range. A failing throttle body only gets jerkier and the stalls get worse, so swapping the unit and relearning the positions put the throttle response properly right.