The brief
The A4 had reached 120,000 km, the recommended timing belt interval on this engine, and the owner brought it in for the change on schedule rather than waiting for a noise or a hard start. The right call, because on this engine a snapped timing belt takes the valves with it. The timing belt keeps the camshaft and the crankshaft turning in step, so the valves open and close at exactly the right moments, and on this engine it also drives the water pump. It's a service item with a set life, and over time the belt goes brittle and develops cracks, with the tensioner and the idler and pump bearings wearing alongside it. Wait for one of them to fail and you risk bent valves, damaged pistons or worse, so the job is preventive, done by the clock as a full kit.
The diagnosis
With the covers off, the old belt was glazed with the start of fine cracks, exactly what a belt at this mileage looks like, the tensioner had play in its bearing, and the water pump, which this belt drives, was just starting to weep at its shaft seal. Doing the belt and leaving the pump and the worn tensioner would mean the car back in soon and you'd pay the labour to get back in there twice, so it was the standard kit job, belt, tensioner, idlers and pump, all together.
The work
The timing was locked with the proper tools so nothing could move, and the old belt, tensioner, idler pulleys and water pump all came off. A new genuine Audi-spec timing kit went in, the timing set back to spec on the tools, and the cooling system refilled with the correct coolant. Then the engine was run through a warm-up cycle to confirm the belt drive was silent and the pump was dry, followed by a road test.
The outcome
Clean timing reset, a dry water pump, a silent belt drive across the rev range, and the biggest single failure point on this engine taken off the table. The A4 went home good for the next belt interval. A timing belt gives you no warning when it snaps, so spending the money on the full kit on time is the cheap insurance against a far bigger, far more expensive repair later.