SM3 NOx Catalyst Aging Exhaust Leak

Greetings, I had a similar problem with my 2014 Opel Astra. The NOx sensor was faulty because of a leak in the exhaust system. Have you checked your exhaust manifold for cracks or loose connections? It could very well cause both the leak and catalyst inefficiency.

Thanks for the tip! Where exactly on the exhaust manifold did you find the issue? Was the leak clearly visible?

The leak was near the flex pipe on mine. It wasn't immediately obvious, but I could hear it hissing when the engine was cold. Given the error code and your exhaust leak, I would suggest having a workshop pressure test your exhaust system. It's likely the culprit.

I appreciate the insight. I took it to the workshop, and it turned out to be a cracked exhaust manifold, just as suspected. Repair cost was 265€. Thanks again!