Emissions issue

I have a 2006 BMW 750li that has 94000 miles on him. I recently took him to get a smog and he failed. I was told by a BMW repair shop that my car had a valve stem problem that was threw my catalytic converter and that I need major work done to my vehicle that drives so smoothly????? Then he finished it off by saying.... It's just a problem with that series of car (if I had a nickel for every time I heard that). Help.... He made no damn sense to me.

What was the failure for? Smoke, high hydrocarbons ..... etc.

He said smoke made a rich content...