A couple of weeks back, the CEL popped up on son's 2018 420i. I checked for fault codes, none was stored. A search on the BMW forums indicated that it’s not uncommon to get a CEL with no codes on the F30 cars, and that it often simply resets itself.

No problem, it was due for its first service anyway, so booked it in and mentioned the CEL had been lit for a couple of days. Of course, the day after booking the service, the CEL goes out and doesn't return.

Jump forward to today, it has its service while my lad waits. When it was completed, the service guy told him that during the service inspection they found the emissions were outside the normal range. The oxygen sensors were checked and were fine, so they did an endoscope inspection in the exhaust which showed the catalytic converter’s matrix had partially broken up internally.

There was no external damage or signs of impact to the cat can, so it’s being replaced under warranty. Strikes me as an unusual fault though, I haven’t heard of many cases of cat matrix failure.