To get your cat hot enough to work properly you need to sit in the car with revs held at 4-5k for about 15 mins! Mine has the full FRP exhaust and after that warm up it passes every time, otherwise it will just keep failing! It feels like the car will explode, but it won't, the rad fan may kick in a few times but that is fine, don't waste money bodging in a cat just try this first, the point of the racing cat is to not restrict flow, the result being it needs to be almost glowing to actually work! Cat cleaner or redex etc will not fix the problem as nothing is blocked