Yep leaking injectors. Just had ours replaced on the 335i - symptom was tied to cold days, cold engine. Once it warmed up all was good - for a while. After a month of putting up with it, we'd get CEL or limp mode. Eventually enough rich exhaust went downstream it clogged up one cat and that exploded down-stream trashing the exhaust with honeycomb material.
Only way to check the injectors, I believe, is to bench test them and new ones are not cheap. Good luck!