The brief
Miss Santhi brought her A4 in for its routine service, with a check engine light on the dash. Two things in one visit: the service, and the scan to find what was behind the light. A service runs through the whole car, and the diagnostic scan that comes with it reads any stored faults. The light traced to the intake air temperature sensor, the small sensor that tells the engine computer how warm the incoming air is so it can trim the fuelling. When that sensor reads wrong, the fuelling goes slightly off and the management trips the light. It's a small, common fault, a quick fix done with the service, and a faulty sensor doesn't recover, so it needed replacing.
The diagnosis
The service scan pulled the fault to the intake air temperature sensor, the reading erratic and not to be trusted, which is exactly what would trip the light. The rest of the engine and the intake checked out, and the routine service items, oil, filters, fluids, were due alongside. So it was a routine service plus an intake air temperature sensor: the wear items refreshed, the fluids freshened, a fresh sensor, the code cleared.
The work
The engine oil and filter were changed, the air and cabin filters replaced, the fluids topped. The faulty intake air temperature sensor was removed and a new genuine Audi-spec sensor fitted, the fault code cleared, and the engine's fuelling adaptations reset so it could relearn against a sensor reading the real air temperature. A full diagnostic scan confirmed no codes left. A road test confirmed a clean start, steady running, and the check engine light staying off.
The outcome
A clean bill of health on the A4, fresh oil and filters, fluids topped, the check engine light gone, and the running back to clean. The car went home serviced and sorted. The check light was a faulty sensor, caught and changed during the service, so it left right in one visit.
