Engine Mechanical problem
2000 Vauxhall Corsa Manual 43000 miles

I have always had problems with my engine management light, but recently it has worsened and comes on most of the time. As before only came on once too much pressure was put on the accelaratore. Now it comes on when stood still, setting off and any random time. I have had this checked several times and was told it was a sensor, got the sensor replaced had all sensors checked through the dyagnositc machine and apparently was ok. But it is not. What would your advice on what to do about this and why it keeps coming on and going off.