Renault Scenic Exhaust Treatment Malfunction

Sounds familiar! I had a similar nightmare with my old Saab 9-3. Check Engine Light, the whole shebang. Rough running? Strong exhaust smell? I was convinced it was the catalytic converter too. Have you checked the lambda sensor (oxygen sensor)? That was the culprit in my case. Apparently, when those go bad, they can throw off the whole exhaust system's readings. Do you get error codes with a scanner?

Lambda sensor, eh? I haven't checked that specifically. Where is that located, and how would I know if it’s gone bad? And yes, I got a code, P0130. Is that pointing in the lambda direction?

P0130 is definitely related to the O2 sensor circuit malfunction, so yes, that's your lambda. On my Saab, it was near the catalytic converter – probably in a similar spot on your Scenic. Honestly, unless you're comfortable with a multimeter and some wiring diagrams, it's probably best to get it checked by a decent workshop. Faulty lambda sensors can also damage the cat, so it is better to get it checked sooner rather than later. Don't get ripped off though, ask for a quote before going ahead with the repairs.

Thanks a bunch for the insight! I took it to a mechanic, and you nailed it – it was the upstream lambda sensor. Cost me 375€, including the sensor and labor. Engine's running smooth again, and the exhaust smell is gone. Really appreciate the help!