Agreed !

The code will more than likely be P0420 cat efficiency or summit like that , basically the ford ECU isn't seeing the numbers it should be from the standard cat (which is huge) - had the same on my ST with a 100 cell cat I just lived with it .

Wouldn't keep pulling KAM fuse to trick the car into thinking it's ok , get a sensor spacer as above and try that , failing that a good remap on the car will put the light out as staffy owner said it will be optimised for any performance mods . Pretty sure the usual suspects could do this for you , Jamsport , REVO , Collins , etc etc