The brief
The A4 had its check engine light on, and the scan came back with a code pointing at the oil level and pressure sensor. He brought it in. That sensor sits low in the sump, screwed in through the pan, and it reports the engine oil level and the oil pressure to the car's computer. When it fails, it sends bad or no data, so the management can't trust the oil readings and lights the dash, and you can get false oil warnings or a missing oil level display. The sensor also has a seal where it passes through the sump, and that can weep oil with age. A faulty sensor doesn't recover, so it needs replacing, with a fresh seal while it's out.
The diagnosis
A diagnostic scan confirmed it, the fault code pointing at the oil level and pressure sensor, the reading erratic and not to be trusted. The oil pressure itself, checked properly, was fine, the engine was healthy, it was the sensor at fault, not the oil system. That's a sensor replacement, you don't repair it, so the call was a new genuine sensor with a fresh seal, fitted and the code cleared.
The work
The undertray came off and the oil was drained enough to get at the sensor, the old oil level and pressure sensor removed, the seating cleaned up, and a new genuine Audi-spec sensor fitted with a fresh seal, torqued to spec. Fresh oil to level, the fault code cleared, and the engine run and checked warm for any weep. A road test confirmed the check engine light stayed off, the oil readings were correct, and the area was dry.
The outcome
No check engine light, the oil level and pressure reading correctly, and the sensor sealed and dry. The A4 went home with the fault sorted. A faulty oil level sensor leaves the management without reliable oil data and can throw false warnings, so renewing the sensor and the seal put the reading right and cleared the light at its source.