Smart Forfour Exhaust Leak and Cat Converter Warning

Had similar symptoms on a 2015 Smart Fortwo. The engine warning light came on and performance dropped significantly with rough idle. Initial diagnosis revealed a failing oxygen sensor, but further inspection showed exhaust leaks around the catalytic converter mounting points. The main repair involved replacing both the upstream oxygen sensor and addressing multiple exhaust leaks. The exhaust manifold gasket had deteriorated, allowing fumes to escape before reaching the catalytic converter. This explained the strong exhaust smell and poor engine performance. This turned into a more complex repair than initially expected. The catalytic converter needed to be removed to properly access and fix all leak points. The job also revealed early signs of catalytic converter degradation, though it was still functioning within acceptable limits. The rough running was primarily caused by the faulty oxygen sensor sending incorrect fuel mixture data to the engine computer. Once replaced, the engine ran smoothly again and the exhaust leaks were eliminated. It was crucial to fix this promptly as prolonged exposure to exhaust fumes can damage other engine components. Best approach is getting a proper diagnostic scan first, then having a thorough inspection of the entire exhaust system from manifold to tailpipe. The symptoms you describe strongly suggest multiple issues that should be addressed soon to prevent further complications.

Thanks for sharing your experience. I am dealing with similar issues on my Smart Forfour right now, complete with check engine light and rough running. I also noticed the NOx sensors needed attention during diagnostics, alongside the oxygen sensor problems you mentioned. The exhaust pipe concerns you described match exactly what my mechanic pointed out. Did you end up having any issues with the EGR valve during the repair process? Also, how has the catalytic converter been holding up since the fix? Would really help to know what the total repair cost was in your case, so I can budget accordingly. Have you experienced any related problems since getting it fixed?

Great to provide an update on my Smart Fortwo repair. After getting it fixed, the Exhaust treatment malfunction warning completely disappeared. The main culprit was indeed a defective Lambda oxygen sensor, which cost 375€ to replace. The catalytic converter has been working perfectly since the repair, and I have not experienced any additional EGR valve issues. Running diagnostics showed all NOx and oxygen sensor readings are now within normal parameters, and the engine performance has returned to normal with no rough running. The exhaust leaks we initially found were properly sealed during the sensor replacement, and there have been no recurring problems. Regular sensor readings from the diagnostic system confirm everything is working as it should. If your symptoms match what I experienced, focusing on the oxygen sensor might be the most cost-effective first step before exploring more extensive repairs to the catalytic converter or EGR system.

Thanks everyone for the helpful input. Just wanted to close this thread with my repair experience. After taking it to a new mechanic, they found multiple issues. The exhaust pipe had several small leaks, and both NOx sensors were failing. The engine light turned out to be primarily triggered by a faulty oxygen sensor, similar to what others mentioned. Total repair came to 850€, more than expected but worth it. They replaced both NOx sensors, fixed the exhaust leaks, and installed a new oxygen sensor. The catalytic converter thankfully only needed cleaning, not replacement. The car runs perfectly now, no more rough idle or exhaust fumes. Engine light is off and performance is back to normal. Definitely learned my lesson about addressing these issues quickly before they cause more extensive damage. The new mechanic was much more thorough than my previous one, taking time to explain each problem and showing me the faulty parts. Really glad I switched repair shops and got a proper diagnosis this time.