will fault codes find the problem ?

hi i own a astra bertone 2.2i and have a engine managment light coming on, it only comes on sometimes and stays on untill i stop the car and restart it . the car is losing power when this light comes on both to engine and electronics ie windows will not go up on there own i have to keep my finger on the button when light isnt on they go up on there own from 1 touch. when driving with this light on it only seems to slug in 3rd i just want to know will the fault code reader find out wat this problem is vauxhall want to charge me £90 just to check the fault code so just wondering if any 1 has had the same sort of proplem cheers