350king

Hi, I have had the engine management light come on and a mobile diagnostic guy has come up with the same fault after hooking the car up to his laptop system. The light went out but last night it has come on again :-( have you any idea how much this will cost to replace? Is it a big job? Any help appreciated as I am just in the process of trying to sell it to a car buying website and typically this has to go and happen and they will no doubt pick up on it and change the price. Cheers