Exhaust Smell and Rough Running Points to VW Cat Issues

I had a similar issue with my 2020 VW Passat last winter. Having some experience with exhaust system problems, the symptoms you describe match what I encountered. The root cause in my case was indeed a faulty lambda sensor (oxygen sensor). The repair cost me 375€, including parts and labor. The workshop diagnosed it using their diagnostic tool, which showed the sensor was giving inconsistent readings. This explained both the check engine light and the rough running, especially during acceleration. What made me suspicious about an exhaust leak initially was that strong exhaust smell, but the technician confirmed the exhaust system itself was intact. The faulty sensor was causing the engine to run rich, leading to that strong fuel smell from the exhaust. To better assist with your specific case, could you share: Current mileage on your Jetta, Whether the check engine light is solid or flashing, If you notice any decrease in fuel efficiency, Whether the car has any performance modifications These details would help determine if your issue matches what I experienced. The symptoms could also point to other components like the EGR valve or an actual exhaust leak, so having more information would be helpful for a more accurate comparison.

Last service was done at 115000km. The engine light is solid, not flashing. My fuel consumption increased by about 20% since these issues started. After checking the exhaust pipe more carefully, I noticed black soot around one of the connections near the NOx sensors. When the engine is cold, the smell is much less noticeable, but after warming up, the exhaust gets quite smelly. The malfunction always occurs in the same pattern, rough idle after a cold start, then gets better, but returns under load. The particulate filter was replaced under warranty at 90000km, so I hope its not related to that again. Running completely stock, no modifications. Just looking for some guidance before taking it to the shop next week.

Thanks for those additional details. The symptoms you describe are slightly different from what I experienced with my Passat. While my issue was primarily sensor-related, your black soot deposit and the specific behavior pattern suggest something more complex. The fact that the particulate filter was already replaced once is significant. However, your current symptoms point more toward a potential exhaust leak at the connection point near the NOx sensors. The black soot accumulation in that area is a clear indicator. In my case, when the exhaust leak developed later on my Passat, it showed similar symptoms, especially the cold start behavior and increased fuel consumption. The catalytic converter efficiency was impacted due to incorrect exhaust flow readings from the compromised system. The key differences from my initial sensor problem are: Visible soot deposits, Strong correlation with engine temperature, Consistent behavior pattern, Higher fuel consumption Given these signs, I recommend having the exhaust system connections inspected first, particularly around those NOx sensor mounting points. My repair involved replacing a damaged gasket and retorquing the connections, which cost about 250€. Much simpler than the full sensor replacement I needed earlier. Would be good to know if you hear any ticking or hissing sounds from that area when the engine is running.

Thanks for the input. After reading these responses, I feel better prepared for my shop visit next week. The black soot near the NOx sensors definitely seems to be a key symptom that wasnt obvious to me at first. And yes, now that you mention it, there is a slight ticking sound from that area when the engine is warm. Since my Jetta already had a particulate filter replacement, I will make sure to mention the exhaust pipe connection issues to the technician. The rough running and malfunction pattern seems very similar to what others experienced with exhaust leaks. Looking at my warranty paperwork, these components should still be covered since the car is relatively new. Will update once I get the official diagnosis, but at least now I can have an informed conversation about the catalytic converter system and surrounding components when I take it in. Hopefully its just a connection issue like some of you experienced and not a major repair.