engine light on 2003 BMW 325i

engine service light is on, what could the most basic problem be?

Asked by for the 2003 BMW 325i
Check out the RepairPal article on the Check Engine Light - and the page for your car -

That should get you started. The light indicates that there is a failure in your emission control system, and it could be anything from a loose gas cap to a failing sensor.
1 more answer
The most basic thing could be a loose or bad fuel cap. Once I replaced mine, it went away.

Another could be a miss-fire. How long has it been since you changed plugs? Did you use the best plugs?