Smart Fortwo NOx System Failure Causing Exhaust Leaks

I encountered similar symptoms on my Smart ForFour CDI from 2006. The issue turned out to be a significant exhaust leak where the catalytic converter connects to the exhaust manifold. The NOx emissions system warning was triggered because the sensors were getting incorrect readings due to the leak. The car diagnostics revealed multiple issues: a cracked flex pipe, deteriorated gaskets, and corroded mounting brackets. The exhaust leak was allowing harmful gases to escape before reaching the catalytic converter, affecting both fuel efficiency and air quality readings. The repair involved replacing the entire front section of the exhaust system, including the catalytic converter assembly and NOx storage catalyst. This was not a quick fix, the components needed special ordering and the work took considerable time due to rusted bolts and tight access points. This is typically a serious issue that should not be postponed, as it can lead to: Further damage to the emission control systems, Failed emissions tests, Potential damage to other engine components, Unsafe exhaust gas exposure Would recommend having a specialized diesel technician perform a complete exhaust system inspection, particularly focusing on the area between the turbocharger and catalytic converter where leaks commonly develop.

Thanks for sharing your experience. This sounds exactly like what I am dealing with. My symptoms match perfectly, especially the leak around the catalytic converter area. I will definitely have it checked by a diesel specialist as you suggested. Would you mind sharing what the total repair cost was in your case? Also curious if you have had any related issues pop up since the repair was done with the exhaust system or NOx catalyst?

Since my previous post, I had the repair done by a certified diesel specialist and can confirm it solved the NOx emissions issues completely. The total cost came to 265€ for fixing the exhaust system leak, which was significantly less than I initially feared. Car diagnostics confirmed the NOx storage catalyst aging was causing the problem, but once the leaks were properly sealed, all systems returned to normal operation. The fuel efficiency has notably improved since the repair, and I have not seen any exhaust fumes or warning lights in the past months. The automotive repair shop did a thorough job sealing all connection points, and I have had zero related issues since then. It was definitely worth addressing the problem promptly rather than risking more extensive damage to the emissions control components. The car runs much smoother now, and regular checks show all emissions values are staying within normal ranges. Just make sure you find a shop that specializes in diesel emission systems for the best results.

I visited a certified diesel specialist last week based on the helpful advice here. They found the exact issue, a significant leak where the catalytic converter connects to the exhaust system. The diagnosis confirmed problems with the NOx storage catalyst and several deteriorated seals. The repair cost ended up at 290€, which included fixing all leaks and replacing the worn components. The fuel efficiency has already improved noticeably, and the exhaust fumes are completely gone. The check engine light turned off right after the repair. The emissions system is now working perfectly, and a follow-up test showed all values are back to normal. Really glad I addressed this promptly instead of risking more damage to the engine and air quality systems. The whole repair took about 4 hours, and the mechanic thoroughly explained all the work they performed. For anyone facing similar issues, definitely seek out a specialist who knows diesel emissions systems. The investment was worth it considering the improved performance and peace of mind knowing the car is running safely again.