2002 Vauxhall Vectra engine management light staying on

My car when I was driving home one night sounded like a tractor and lost power (no lights on dashboard though). I managed to get the car home and my mechanic came round to see what was wrong. After looking at it he said he thought it was the coil pack and ordered one for me. He ordered a patterned one which cost me £150 but when he plugged it in and drove it the misfiring etc had cleared but the engine management light came on and the message said something about the cam being off balance or something. When my mechanic checked with Vauxhall and he knows my car it has a timing chain so knew that this was a wrong message. In the end he exchanged it for an original Vauxhall part which cost an additional £86 but the previous message from the diagnostic report cleared. However the light still comes on and the message now is completely different and says it has low emissions from sensor 1. It drives about 35 miles before coming on after he clears it and it has been coming on every time but not immediately. He then checked and on his computer it says check for any leaks or blows in the exhaust system. I got this checked and I had a blow in the middle section so ended up getting the middle and back box renewed at a further cost of £120 and thought the problem may be fixed. Drove 35 miles again and the light has come on again> My mechanic has spoken to several garages and they have said give it a good blow out but I really don't want to go down the road of spending another £200 on a catalytic converter when it could be something silly like a complete reset of the ECU or something. The funny thing is I never got the management light on when the initial problem started and it's only since the parts have been fitted that they have come on. My mechanic is convinced that there is nothing wrong with the catalytic converter as there is no smells from the exhaust and he said it is like fresh air coming out of of there. Can anyone help????