The brief
The A3 had crossed 120,000 km and the owner wanted a proper service with the timing belt looked at, which is exactly the right call at that mileage. The timing belt is the one wear item you don't gamble on, when it lets go the engine usually goes with it. The timing belt synchronises the crankshaft and the camshafts so the valves open and close in time with the pistons. It's a rubber belt, it ages and wears, and a worn one can jump teeth or snap, which on these engines means the valves and pistons meet, and that's a rebuild. So the manufacturer sets a replacement interval, and at 120,000 km this one was on it. A proper job changes the belt with its tensioner and rollers as a kit, not the belt alone, and gets done alongside the rest of the service while everything's apart.
The diagnosis
A check confirmed the timing belt was due on age and mileage, with the tensioner and rollers that run it tired alongside it, and the regular service items, oil, filters, fluids, were due too. The engine was running fine, this was preventive: do the belt kit before it could fail, and the full service with it. So it was a major service plus a timing belt kit: the belt, tensioner and rollers together, the wear items refreshed, and a clean diagnostic scan.
The work
The timing belt, tensioner and rollers were replaced as a genuine Audi-spec kit, the engine timing checked and set exactly to the marks, the water pump checked over while everything was accessible. Alongside it the engine oil and filter were changed, the air and cabin filters replaced, the fluids topped, the brakes and underbody checked, and a full diagnostic scan run with no codes left. A road test confirmed the engine ran smooth and quiet, timed properly, with nothing leaking.
The outcome
A fresh timing belt kit timed properly, a full service done alongside it, and the A3 back on the road with the one part you can't afford to ignore renewed for the next interval. The car went home sorted. A timing belt past its interval is the cheapest part with the most expensive failure, so doing the kit and the service together is the proactive job that keeps a 120,000 km car a reliable one.