RCZ NOx Catalyst Error and Exhaust Fumes

I had a similar problem with my older Opel Astra. Same symptoms: check engine light, error memory filled with NOx catalyst codes. Different engine, different car, but the root cause might be similar. Have you checked for any leaks in your exhaust system?

A leak? Where should I be looking, exactly? I'm not super familiar with the exhaust system layout on these cars.

It's difficult to pinpoint without seeing it. Common areas are around joints, welds, or any flex pipes in the exhaust system. Given the NOx catalyst issue, any leak upstream of the catalyst is suspect. It's best to have it inspected at a workshop, to be honest. It's quite common and can cause additional damage if left unresolved.

Took it to a shop, and you were spot on. Small crack in the exhaust manifold. Replaced it, cleared the codes, and everything seems fine now. Cost me 265€. Appreciate the help!