Mini Cooper Exhaust Treatment Malfunction

Given the symptoms you describe – engine light, rough running, exhaust leak, and strong smell – a defective lambda sensor is certainly a plausible culprit. However, "exhaust treatment malfunction" is a rather broad description. Could you provide the exact error code you retrieved? This will help narrow down the possibilities. Furthermore, has the catalytic converter's performance been evaluated recently, as a malfunctioning converter can sometimes mimic lambda sensor issues? Finally, have you checked for any exhaust leaks before the catalytic converter? These can also lead to similar symptoms.

Thank you for your reply. I do not have the error code at hand. I have not evaluated the performance of the catalytic converter. Regarding exhaust leaks before the catalytic converter, how would I check this?

Given the absence of the specific error code, a physical inspection is indeed necessary. To check for exhaust leaks before the catalytic converter, visually inspect the exhaust manifold and piping for any signs of cracks, rust, or holes. You might also try feeling around the joints while the engine is running (exercise extreme caution to avoid burns). However, diagnosing lambda sensor issues can be complex, as they often interact with other components like the NOx sensors and the catalytic converter itself. A faulty oxygen sensor can wreak havoc on the engine's air-fuel mixture, leading to incomplete combustion and the smell you describe. Based on your description, I would advise taking the vehicle to a qualified mechanic for a proper diagnosis and repair. It is likely a lambda sensor issue, but a professional evaluation is warranted to rule out other potential problems.

I appreciate the thorough explanation. As suspected, the garage confirmed a defective lambda sensor. The exhaust treatment malfunction is now resolved; the engine light is off, and the strong exhaust smell is gone. Total cost was 375€.