I have a 1.6 litre petrol Vauxhall Zafira. I recently put it in for it's MOT and it's failed on two minor things and then co2 emissions. I tried additive and a good thrashing and put it for retest it still failed. So I tried an oil change and oil filter gave it a run again and it still failed. So put it for diagnostics which the garage said came back clear so they were sure it was the cat. So i had a new cat put in and downpipe at the garages recommendation but had my brother in law do it independently who is a qualified mechanic. When I took it for a retest however it still failed on co2. Garage who did the diagnostics say it's because the cat was bought from a cheap supplier (Euro car parts). Does anyone have any idea where I go now? Don't wanna pay for the cat to be replaced again. Surely even poor quality parts couldn't be sold unless they pass? Is the garage fobbing me off as they assured the cat change would fix it? Any help would be greatly appreciated. (Also in terms of emmissions MOT test centre said it shoots up to 2% then after running drops to 0.34% then fluctuates up and down from there, but never lower then the 0.2% needed.)

Thanks John.