Vauxhall astra GSI mk3 2.0 ltr 16v Help!?

My boyfriends car recently went for MOT sailed through everything but failed emmisions. I'm told there are a few possibilities such as water temp, sensor (wiring breakage) that controlls the injection as well as the idle control valves but as he has checked both of these he is wondering if anybody has either come across a reason causing failure of emmisions or knows of a possibilty to solving this.