A8 Hybrid Exhaust Issues Point to Failed Catalyst

I had a very similar issue with my 2018 A6 Hybrid last year. Having worked on basic maintenance tasks before, I recognized these symptoms immediately but needed professional help for this one. The rough idle and exhaust smell turned out to be caused by a faulty oxygen sensor, not the catalytic converter as I initially suspected. The workshop diagnosed it using their diagnostic equipment, which showed the lambda sensor was sending incorrect readings to the engine control unit. The repair cost me 375 Euro including parts and labor. After replacement, both the rough running and exhaust smell disappeared completely. The engine light also went off and stayed off. Before jumping to conclusions about the exhaust system or particulate filter, its worth getting the oxygen sensor checked first, as its often the simpler and less expensive fix. To help provide more specific advice about your case, could you share: Current mileage on your A8, Whether you notice any performance issues or increased fuel consumption, If the check engine light is steady or flashing, Whether youve recently had any other exhaust system work done These details would help determine if your issue matches what I experienced or if its something different in the exhaust treatment system.

Just had my 134743km service done last month and unfortunately experienced the same symptoms. The exhaust leak turned out to be a cracked flex pipe, not the catalytic converter as initially suspected. The oxygen sensor readings were erratic due to the leak affecting the air-fuel mixture. The repair bill was 850 Euro complete, as they also found the DEF fluid system needed attention during diagnosis. The engine light cleared after repairs and the rough running is completely gone. For reference, the complete fix included: Flex pipe replacement, DEF system service, New gaskets and hardware, Diagnostic testing Would suggest getting a smoke test done to confirm if you have a similar exhaust leak before replacing more expensive components.

Thanks for the update on your A8 repair. Having gone through similar issues with my A8 Hybrid, I can confirm exhaust leaks near the flex pipe are a known weak point. My symptoms matched yours exactly, the rough running and strong exhaust smell were telltale signs of the leak affecting the exhaust treatment system. In my case, the NOx sensors were also giving false readings due to the exhaust leak throwing off the system measurements. The repair shop found the flex pipe had cracked right before the catalytic converter, which explained why the engine was running rough. The total repair came to 890 Euro in my case, as they also needed to clean the EGR valve which had become clogged due to the improper exhaust flow. After replacing the flex pipe and servicing the EGR system, all issues cleared up completely. A quick tip for others: if you notice exhaust smell combined with rough running, have the exhaust pipe connections checked first before assuming its a more complex issue with the particulate filter or catalytic converter. The simpler fix often solves the problem.

Thank you for sharing those details about the flex pipe repair. The exhaust leak makes perfect sense given the symptoms, and I am glad I got it properly diagnosed rather than jumping to conclusions about the catalytic converter. The DEF system service was an unexpected but necessary addition, good catch by the shop. Speaking of my original symptoms, I should mention that the rough running was most noticeable during cold starts and gradually improved as the engine warmed up. This ties in with what we now know about the exhaust leak affecting the air-fuel mixture readings. For anyone else searching with similar symptoms: get the exhaust system smoke-tested before replacing expensive parts like NOx sensors or the particulate filter. My total repair cost of 850 Euro was much better than the 2000 Euro+ initially feared for major exhaust treatment system work. The car now runs perfectly smooth again and there is no trace of exhaust smell. Regular checks of the flex pipe area during service should help catch any future issues before they become serious problems.