Audi A5 Diesel: Jerky Acceleration & Engine Light Fix

Having some experience with diesel engines, I faced a very similar issue with my 2013 Audi A4 diesel. The symptoms match exactly what you described, engine light, jerking during acceleration, and rough idle when cold. After initial diagnosis, my workshop found a leak in the exhaust system which was causing incorrect readings from the oxygen sensor. This confused the engine management system, leading to those performance issues. The total repair cost was 265 Euro, including parts and labor. The diagnostic steps taken were: Complete OBD scan, Smoke test of the exhaust system, Testing oxygen sensor readings, Visual inspection of all exhaust components Before jumping to conclusions about the crankshaft sensor or fuel pump, it would be helpful to know: Current mileage on your A5? When was the last timing belt change? Any recent maintenance performed? Does the problem get better after the engine warms up? Any unusual exhaust smoke color? This information would help pinpoint if your issue matches what I experienced or if it might be something different.

Last service was done at 120253 KM. After checking the service records, I noticed the timing belt was changed at 95000 KM. The problem does improve slightly when warm, and there is dark smoke occasionally. The engine misfire seems most noticeable during cold starts, especially in the morning. I checked the fuel consumption and noticed it increased by about 15% since these issues started. Given the mileage and symptoms, I am starting to suspect the oxygen sensor might indeed be the culprit, especially with that dark smoke. I will have the workshop check the exhaust system and oxygen sensor readings first, as that seems like a reasonable starting point before investigating the fuel pump or other components.

Thanks for providing those details. After fixing my A4 with similar symptoms, I can better confirm that our issues sound almost identical. The dark smoke and increased fuel consumption you mentioned are classic signs of an oxygen sensor issue, which matches what I experienced. Based on your timing belt change at 95000 KM, that component should still be fine. My fuel consumption also increased by around 12-15% before the repair, and I had the same dark smoke symptoms during cold starts. In my case, the faulty oxygen sensor was causing the engine control unit to deliver incorrect fuel mixtures, especially during cold starts. After replacing the sensor and fixing the minor exhaust leak, both the smoke and jerking disappeared completely. A failing oxygen sensor often causes: Uneven engine running, Poor fuel economy, Dark exhaust smoke, Performance issues during acceleration The workshop should definitely start by checking the exhaust system and oxygen sensor readings. In my experience, its better to rule out these relatively simple fixes before investigating more complex issues with fuel injectors or the crankshaft sensor. Keep us updated on what the workshop finds. The repair should be straightforward if its the same issue I encountered.

Based on your feedback and the workshop inspection recommendation, I will have them perform the exhaust system check and oxygen sensor testing next week. The symptoms you described with your A4 match my situation very closely, especially the fuel consumption increase and dark smoke during cold starts. The timing belt replacement at 95000 KM was relatively recent, so that should not be the issue. Given the mileage and symptoms, focusing on the oxygen sensor and exhaust system first makes the most sense before investigating the fuel pump or other major components. Will update the forum once I get the diagnostic results back from the workshop. Really appreciate the detailed explanation of your experience and the systematic approach to troubleshooting. This gives me a much clearer direction for the repair process instead of just out of nowhere replacing parts.