S3 Rough Idle + Exhaust Leak Points to Cat Issues

Having some experience with similar issues, I dealt with nearly identical symptoms on my A3 2.0T from around the same generation. The engine light combined with rough idling led me to suspect several issues, but it turned out the oxygen sensor (lambda sensor) was the main culprit. The exhaust leak smell was actually connected to this issue, as the faulty sensor caused incorrect fuel mixture readings, leading to poor combustion and increased emissions. Initially thought it was a cracked exhaust pipe too, but the workshop diagnostic revealed the real problem. The total repair came to 375 Euro, including parts and labor. The fix made a huge difference, smoother idle, no more exhaust smell, and the engine light cleared up immediately. Before jumping to conclusions about the catalytic converter, it would help to know: What year is your S3? Have you noticed decreased fuel efficiency? Did the engine light come on suddenly or gradually? Are there any other warning lights active? Have you had any recent exhaust work done? With these details, I can share more specific insights from my experience. The symptoms you describe are classic indicators of either an oxygen sensor issue or an EGR valve problem, both of which can cause similar symptoms.

After 152890 KM on my Audi, I can confirm the issue was a combination of a failing oxygen sensor and a small exhaust leak at the flange connection. Getting a proper diagnostic scan revealed a malfunction code pointing to the O2 sensor. The repair involved replacing both the oxygen sensor and fixing the exhaust leak with new gaskets and hardware. Total cost ran about 450 Euro, but completely resolved the rough idle and cabin smell. Worth checking your exhaust manifold bolts too, they tend to loosen up on these engines after high mileage, causing similar symptoms. The early 2000s S3 models are known for these particular issues around the 150000 KM mark. Saw significant improvement in fuel economy after the fix, and the engine light hasnt come back since service last month.

Thanks for the update on your S3. Your experience matches mine exactly, had the same issues on my S3 at around 155000 KM. The combination of a failing oxygen sensor and exhaust leak is quite common on these models. After fixing mine, noticed the NOx sensors were also showing early signs of wear during the diagnostic scan. Had them checked as a preventive measure since the exhaust system was already being worked on. The EGR valve looked fine during inspection, but the exhaust pipe flange definitely needed attention, the original gaskets were completely worn out. Good call on mentioning the exhaust manifold bolts. Mine were indeed loose and contributed to the exhaust leak. One tip that worked well: had the workshop check all exhaust system mounting points while they were under there. Found a couple of worn rubber hangers that could have caused issues later. The repair cost on mine was similar, around 425 Euro. The fuel economy improvement basically paid for the repair within a few months. Running perfectly smooth now at 170000 KM with no check engine light or exhaust treatment malfunction warnings.

Thanks for your detailed reply about the oxygen sensor and exhaust leak. After completing the repair last month, I can share some final numbers and results that might help others with similar issues on their S3 2.0T. The total repair came to 450 Euro, covering both the oxygen sensor replacement and fixing the exhaust leak. The mechanic found the exhaust manifold bolts had indeed loosened up, exactly as you mentioned, apparently common on these engines with higher mileage. Since the fix, my fuel economy improved noticeably. The engine light stayed off and the rough idle completely disappeared. They also inspected the catalytic converter and NOx sensors during the service, confirming both were still in good condition. For anyone else facing similar symptoms around the 150000 KM mark, definitely worth checking those exhaust manifold bolts and oxygen sensors first before assuming worst-case scenarios with the EGR valve or catalytic converter. Getting a proper diagnostic scan saved me from unnecessarily replacing more expensive components. Really appreciate all the input from everyone, saved me time and money by pointing me in the right direction.