Megane NOx Catalyst Aging

NOx catalyst aging... that's not great. NOx, or nitrogen oxides, are nasty pollutants, and the catalyst cleans them up. The exhaust smell and error code together point to a potential exhaust leak before or around the catalyst. Is the engine louder than normal, or are you noticing any black soot near exhaust joints? Could be a cracked manifold or a blown gasket. Checking your exhaust system is a must!

Okay, exhaust leak makes sense with the smell. But would a small leak really cause the NOx catalyst aging error? I thought those catalysts were supposed to last a pretty long time. And how does that even work?

A leak, even a small one, throws off the whole exhaust gas recirculation (EGR) and catalytic converter system. The engine control unit (ECU) relies on specific readings to properly manage the air/fuel mixture and emissions. A leak means unmetered air, which leans out the mixture and can overheat the catalyst, damaging it over time. The catalyst itself uses precious metals to convert NOx into harmless nitrogen and oxygen. At that mileage, with a leak, it's plausible the cat's efficiency has degraded. You should get it to a workshop because with a leak your car will fail the next emission test. They can properly diagnose the leak and check the catalyst's function. Unfortunately a faulty cat is always very pricey.

I took it to a local garage and they confirmed what you said about the exhaust leak and the NOx catalyst being affected. Turns out there was a crack in the exhaust manifold, which likely caused the catalyst to work harder and age prematurely. Repair cost me 265€ including the new gasket.