Peugeot 307 NOx Catalyst Aging and Exhaust Leak

Sounds familiar. I had a similar headache with my old Saab 9-3 a few years back: check engine light, emissions warnings. In my case, the issue stemmed from a cracked exhaust manifold. Are you sure about the leak?

Cracked exhaust manifold, you say? How did you pinpoint that as the culprit? Was it obvious, or did it require some serious sleuthing?

The leak was key. The crack was small, almost invisible, but a smoke test revealed it. The leak messes with the oxygen sensors, throwing off the air/fuel mixture and hindering the NOx catalyst's effectiveness. Seeing as you've got the leak symptom, I would advise you to take it to the workshop.

The workshop confirmed a leak in the exhaust system, as you suspected. Apparently, that was the root cause of the NOx catalyst aging issue and check engine light. After replacing the exhaust pipe, at a cost of 265€, everything is back to normal. Fuel efficiency seems improved too! I really appreciate your advice.