Q8 NOx Catalyst Failure: Symptoms and Solutions

I had a similar issue with my 2017 Q7 last year. Having some experience working on exhaust systems, I first noticed the problem through increased engine noise and error codes. The NOx catalyst and catalytic converter issues can be tricky to diagnose properly. In my case, the automotive repair shop found a significant crack in the exhaust system just before the catalytic converter. The leak was causing false readings and triggering the check engine light. The repair involved welding the crack and replacing a damaged gasket. Total cost was 265 Euro, which included diagnostics and labor. The fix resolved both the error codes and the emissions problems. To better assist with your specific case, could you share: Current mileage on your Q8, Any unusual sounds or smells, Whether the engine performance issues occurred gradually or suddenly, Complete error codes from diagnostics NOx emissions problems can stem from various sources in the exhaust system, so these details would help narrow down the exact cause.

After the latest service at 184837 KM, my Q8 started showing these symptoms more frequently. The engine performance declined gradually over the past few weeks, and I notice a distinct sulfur smell, especially during acceleration. The error codes are P0420 and P0430, pointing to catalyst system efficiency below threshold. The nitrogen oxides readings are above normal according to the diagnostic report. The unusual sounds seem most noticeable when the engine is cold, particularly a rattling noise from the exhaust system area. Based on my research, this suggests potential internal damage to the catalytic converter components. Would these symptoms align with what you experienced before your repair?

Thanks for sharing those details. Your symptoms actually match what I experienced with my Q7 quite closely, though the outcome was slightly different than my initial repair. After my first fix, similar issues returned within a few months. The final diagnosis revealed that while the exhaust leak contributed to the problem, the root cause was indeed a failing NOx storage catalyst. The rattling noise you mentioned from the exhaust area was particularly familiar, in my case, it indicated internal degradation of the catalyst substrate. The P0420 and P0430 codes, combined with elevated nitrogen oxides readings, strongly suggest catalyst system deterioration rather than just an exhaust leak. The sulfur smell during acceleration further supports this, as it typically indicates incomplete conversion in the catalytic system. My automotive repair shop ultimately had to replace the entire NOx catalyst assembly, which resolved the fuel efficiency issues and error codes completely. The total cost was significantly higher at 1890 Euro, but the fix has been permanent. Given the similar mileage and symptoms between our vehicles, I recommend getting the NOx catalyst assembly checked specifically. The gradual performance decline points more toward catalyst aging than a sudden failure like my initial exhaust leak.

Thanks for the comprehensive information about your repair experience. Those details about the P0420 and P0430 codes really help clarify things. Your description of the sulfur smell and gradual performance decline matches my situation exactly. The car diagnostics report from my last service combined with these symptoms seems to confirm the NOx catalyst is likely failing. This makes more sense than just an exhaust leak, especially considering the timing of the issues after reaching this mileage. Based on your experience with the temporary fix versus the complete catalytic converter replacement, I will skip attempting minor repairs and go straight for a full inspection of the catalyst system. While the 1890 Euro cost is significant, addressing the root cause appears more cost-effective than attempting partial repairs. Will schedule an appointment with a shop that specializes in automotive repair and fuel efficiency issues to have the entire NOx catalyst assembly evaluated. The rattling noise and emissions problems need proper resolution before they potentially cause additional damage to the exhaust system.