what can be the cause of an check engine light? How can I check?

This car has literally hundreds of codes for the check engine light and there are dozens of sensors, not just one that can get oily. So, best to have a good German car shop look at your BMW, this car has more than 30 computers, So unless your are trained, don't mess with it or guess by trying parts. Doing that can ruin things and get very,very expensive!