Audi RSQ3 Rough Idle and Exhaust Leak Diagnosis Guide

Had a similar situation with my 2018 Audi RS3. The symptoms matched exactly, rough engine, check engine light, and strong exhaust smell. The initial diagnosis revealed multiple issues, primarily a faulty Lambda sensor combined with a damaged catalytic converter. The catalytic converter showed severe internal damage, likely from running too rich for an extended period. The NOx sensors also needed recalibration after the repair. The fault turned out more serious than expected since the damaged catalytic converter had started affecting the EGR valve operation. Key things to have your mechanic check: Complete exhaust gas analysis, Lambda sensor voltage readings, Catalytic converter efficiency test, Pressure test for exhaust leaks, EGR system operation Make sure they perform a thorough diagnostic scan before and after repairs. In my case, simply replacing the Lambda sensor without addressing the catalytic converter would have led to recurring issues. Request a detailed inspection of the entire exhaust system, as these components work as an interconnected system. The repair resolved all symptoms and restored proper engine performance. Having the complete system checked prevented further damage to other components.

Thanks for your detailed reply about your RS3 issues. Really helpful since the symptoms match my situation. Did you remember roughly how much the complete repair cost with parts and labor? Also curious if youve had any recurring issues since the fix, especially with the engine light or NOx sensors? My mechanic mentioned the catalytic converter replacement alone could be pricey but its hard to gauge what to expect.

Good news about your exhaust concerns. After taking my RS3 to the mechanic last week, they found and fixed the Exhaust treatment malfunction. The main culprit was a defective Lambda sensor, which cost 375 Euro including parts and labor to replace. Since the repair, the check engine light has stayed off and the engine runs smoothly. No recurring issues with the NOx sensors or any other exhaust components. The mechanic performed a full diagnostic scan afterward to confirm everything was working properly. You mentioned the catalytic converter, fortunately in my case it only needed cleaning rather than replacement, which saved significant cost. A full catalyst replacement typically runs much higher, often above 1000 Euro. This fix proved much simpler than initially feared, but having a thorough diagnosis first was key to avoiding unnecessary parts replacement. The complete exhaust analysis they performed ruled out more serious issues with the EGR valve or other components.

Thanks everyone for the helpful replies. Just wanted to share how my repair went. Finally took it to a new mechanic who actually found the root cause. The exhaust pipe had a crack near the flange, and two oxygen sensors were completely shot. The EGR valve was actually fine, which saved some money. The total repair came to 850 Euro, much less than the catalytic converter replacement I was worried about. They replaced both oxygen sensors, welded the exhaust pipe crack, and did a full system diagnostic. The engine light is now off and the rough running is completely gone. What a relief to have it properly fixed after the previous failed attempt. The engine runs like new again and there is no more exhaust smell. Lesson learned about finding a shop that does proper diagnostics first.