The brief
The A3 had been hunting at idle, the RPM swinging more than it should at a standstill. It hesitated in the first metre off a stop, a beat of nothing before it picked up. And a check engine light was on that would not clear.
The owner had cleaned the throttle body himself, which is a sensible first move, a carboned-up throttle butterfly does cause exactly these symptoms. It helped for a day. Then it came back. When a clean fixes something for a day and then it returns, the cleaning was treating a symptom, not the cause. He brought it in.
The diagnosis
Live data on the scanner showed the throttle butterfly tracking jerkily through its range, not the smooth, predictable movement a healthy throttle body gives. And the codes were a stack of throttle-related ones: throttle valve control system malfunction, throttle actuator control range/performance, throttle position sensor faults.
That picture is an electronic throttle body whose internal actuator, the little motor and gear set that opens and closes the butterfly, is failing. A clean removes carbon, which helps for a while, but it does nothing for a worn actuator. The throttle body itself was the failed component, and on these it is a sealed unit, so the fix is replacement.
The work
Removed the intake hose, unbolted the failed throttle body, and fitted a new VAG-spec replacement. Reconnected the harness.
Then ran the throttle adaptation routine on the scanner, which is essential on these: the ECU has to learn the new throttle body's exact closed position and its full-open position, otherwise the idle and the response are off. Cleared the stored codes once the adaptation was done.
The outcome
Idle steady, no more hunting. No hesitation off the line, the throttle picking up cleanly from a stop. No check engine light return after a full drive cycle.
The A3 went home running cleanly through the rev range. For the owner, that means a car that responds the moment he asks it to, idles steadily at the lights, and has stopped lighting up the dash.
And it is fixed at the cause, the worn actuator, rather than chased with another clean that would have lasted another day.