The Engine Management Light / Check engine light will come on if the ECU has detected a fault in the engine management system. The fault could be many things…a faulty sensor or actuator, emissions too low / high, defective wiring, blocked cat etc, to name but a few. A fault code will then be stored in the ECU to assist diagnosis. The code can be read using a suitable scan tool, which most good local garages now have. A typical cost for this would be around £35 - £50. The code description will guide the technician to a specific area of the car for further investigation. Once the fault is rectified, the EML can be reset with the scan tool. Having a scan done is the most cost effective way to solve the problem, as repairing by part substitution can be very expensive guess work.
Hope this helps