I still have my cat with my milltek cat back and was in for my MOT last Saturday (at my friendly MOT tester). In years gone by I had often been let off the emissions test all together if the computer didn't start etc but this year was different. He had a new more accurate computer to do the testing and mine had to be done three times (with long runs in between to get the cat hotter) before it passed. There was an issue with the lambda emissions and I eventually got it within .004 of the acceptable limit. Was told that my cat was probably on the way out. I can't see how I would have passed without a cat though, but I'm no expert...