hi i have a 2000 corsa lite and the engine control light?

hi i have a 2000 corsa light and the engine control light keeps flashing on and off and now after a wile becz the book said if it only goes on and off i shoulnd pay att to it unless it stays on so now my rev on the engine don't go down when i leave the accelerator i have heard it can be the small black box right to my legs and that i shoul buy a new one i dont know plz sum 1 that know