check engine comes on.

Hello, the only way to find out why the light has come on is to have a professional diagnostic test. Yes, you can get diagnostic tools that can "retrieve codes" but often times that doesn't really give you a good picture of what is going on. A skilled and trained technician will be able to retrieve data and then interpret it to find out what is going on with your BMW. www.repairpal.com/repair-shops Note that many different possible causes can lead to a check engine light - some you won't notice any performance differences with.