There is a problem with the engine management system - could be a loose wire or faulty sensor - you'll need to take to a garage with diagnostic equipment to get the fault code read and either fix it yourself or get them to do it
take it sooner rather than later. I had the same prob with my Focus. The light kept coming on but the car felt fine. When it went in for a check I was advised that I was lucky to have booked i it. The Lamda sensor had been damaged which was effecting my catalytic conevrtor. It cost me �145 to replace, as apposed to �700 for a new catalytc convertor!