gearbox fault codes

am borrowing my mates snap on fault code computer laptop thing tomorrow, what codes am I looking for that would make it flag up a gearbox fault message and go into safe mode? I've spoke to transmission bloke thismorning who says to read the codes see what it thinks is wrong and coz of what I've tried already then reset them and see if it trips them back on when it checks itself.... But I need to know what codes I'm looking for...

any help would be gratefully received thanks guys