as already said alot of owners have fitted de-cats ect to there cars and not had them mapped with no problems,but ive been through 3 engine so id say best to be safe than sorry and if its going to cost you £600 for an ecu?? plus the map £500+ it would just be cheaper to get a simtek fitted o and as for what you should listen out for,id start listening for engine failure sounds like an aerosol can