The brief
Mr Pua's Q5 had an engine check light on, and he wanted a routine service done at the same time and some advice on the light. 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 an oxygen sensor, the sensor in the exhaust that measures the oxygen left in the gases so the engine computer can fine-tune the fuelling and keep the catalytic converter working right. When it ages and reads slow or wrong, the fuelling goes off, economy drops, and the management trips the light. A worn oxygen sensor doesn't recover, so it needs replacing, done with the service.
The diagnosis
The service scan pulled the fault to an oxygen sensor, reading slow and not to be trusted, which is exactly what would trip the light. The rest of the exhaust and the engine checked out, and the routine service items, oil, filters, fluids, were due alongside. So it was a routine service plus an oxygen 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 oxygen 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 oxygen content. 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 Q5, fresh oil and filters, fluids topped, the check engine light gone, and the fuelling back to clean. The car went home serviced and sorted. The check light was a worn oxygen sensor, caught and changed during the service, so it left right in one visit.
