An MOT Fail??

[burnmw]

Large hole in your exhaust system of course it will fail lol

but why?? understand that boxes include baffles and filters but when it's at the back of the system?

*SNIP*

For the same reason that if a spare wheel is present it must be road legal. If it's fitted then they will test it, doesn't matter if you only need one box, if you have more than one and one of them is holey then it will fail.