I don’t think the cat back is causing any issues, I thinks it’s a damaged/faulty sensor. I’m not sure if they’ve even been replaced before I had the car, I’m guessing not as the standard exhaust has been on the car until today and it’s a 10 plate. There’s no leak on the system as the garage who fitted it said themselves. I’ve got a call with Subaru tomorrow to ask about replacement sensors, hopefully that’ll fix the problem.