Lexus LX NOx catalyst aging and exhaust leak

That sounds familiar. I had a similar issue with my '16 Volvo XC90 a while back. The check engine light and a weird exhaust smell were the first signs. Turned out to be a crack in the exhaust manifold, messing with the sensor readings. Have you checked for any obvious leaks or damage under the car?

Interesting. A crack in the manifold, huh? I'll definitely check that first thing in the morning. Was the Volvo also complaining about the NOx catalyst?

Yep, the Volvo was throwing codes related to catalytic converter efficiency, which is basically the same as your NOx catalyst issue. In my case, the leak was causing the engine to run lean, which then damaged the cat over time. Honestly, if you're smelling exhaust fumes and getting those codes, it's probably a leak somewhere in the exhaust system. I'd say take it to a trusted shop to run a smoke test.

Thanks for the tip! Took it to a local shop and they found a small crack in the exhaust pipe right before the catalyst. Welding it up and replacing a gasket cost me about 265€. Check engine light is gone, and it seems to be running smoother now. Appreciate the help!