Seat Inca NOx Catalyst Issue

The symptoms you describe are consistent with NOx catalyst aging, especially given the mileage. Exhaust leaks can definitely throw things off, as the oxygen sensors will report incorrect readings, messing with the air quality readings. It's crucial to rule that out first because a leak affects the entire emissions system. Did the car diagnostics scan point to anything specific regarding the emissions system?

If it's a leak in the exhaust system, how does that cause the NOx catalyst to age faster? I thought the NOx catalyst just deals with the gases after they've already gone through the engine. Is it something to do with the lambda sensors getting bad information?

Yes, lambda sensors are very sensitive, and even a small exhaust leak before the catalyst can introduce excess oxygen, leading to skewed readings. This forces the ECU to adjust the fuel mixture, potentially damaging the NOx catalyst over time. I'd suggest taking it to a workshop to verify if the exhaust system has a leak. This is the most probable cause.

You were spot on. I took my 2000 Seat Inca to a workshop, and they confirmed an exhaust leak was causing the NOx catalyst issue. 265€ later, the emissions system is fixed. Fuel efficiency seems better already, and hopefully I'll pass the next car diagnostics test. Thanks again.