Likely cause of Labmda (Oxygen) emissions being too high

Car: Corsa B (2000 plate) 1 litre engineFailed MOT test (in UK) only due to a lambda reading just over the MOT test threshold. They did the reading twice but both times was just over. Everything else tested was fine with no advisories.Unfortunately the garage were very unhelpful with how to fix the issue. Therefore would appreciate advice on the following questions:1) The garage mechanic vaguely suggested a new "lambda" oxygen sensor would fix the issue, but surely this means the car only failed its MOT because the sensor was wrong, not because the emissions were inappropriate?2) If the new sensor doesn't fix the issue, mechanic said I would need a new cam. Are there no other fixes for an oxygen reading just over the MOT threshold?3) Have had EGR valve issues with the car in the past, and I thought that was the cause. However, the garage said not because CO emissions were zero and the valve is only a few years old. As such, the garage mechanic said the EGR valve is definitely fine - is this correct? via /r/cars http://ift.tt/2tZhF1K