Your BMW's check engine light being illuminated is an automatically smog check failure. Vehicle manufacturers have placed the Check Engine light inside the passenger compartment to inform the driver there has been or is an engine or drive train malfunction. Very often you may not notice an engine problem, but this is only because your engine's computer is operating under pre-programmed information rather then of real-time.
Our recommendation is to have a certified smog check repair shop inspect your BMW. Along with the "Check Engine" light being illuminated, there are stored computer codes in your BMW's ECU (engine control unit) which the smog station will be able to retrieve and use for diagnosing purposes. These codes more then often will direct the smog technician in finding the fault. Once the fault/s (trouble codes) are found the repair shop can begin correcting the faults.