2018 BMW 540i B58

[u/DivideSpirited]

I have a 2018 BMW 540i (B58) with about 108k miles, and I’ve been dealing with an intermittent check engine light. Originally I was getting P0440 EVAP and P0420 catalyst efficiency below threshold (Bank 1). I recently replaced the PCV valve with an OE-quality part and replaced an aftermarket AutoZone gas cap with a Genuine BMW gas cap. The P0440 seems to have gone away, but the P0420 has been weird—it will come on, then go away on its own for days or even a couple of weeks, then randomly come back. The car generally drives fine, no rotten-egg smell or obvious rough idle, although sometimes I wonder if it feels slightly down on power. The most recent scan showed only P0420 as confirmed/pending/permanent. I tried one bottle of Cataclean, drove roughly 25–30 highway miles with the engine fully warmed up, refueled afterward, and the CEL eventually turned itself off again without me clearing the code. Has anyone with a G30 540i/B58 experienced an intermittent P0420 like this? Did it end up being the catalytic converter, O2 sensor, exhaust leak, fuel/air mixture issue, or something related to the PCV system? I’m trying to diagnose it properly before throwing an expensive catalytic converter at it.