Blocked cat or backbox ? if it dont chuck out black smoke then boost pipes are ok ! if no codes then maybe exhaust prob , can you unbolt just the rear box and try without it or is it a one piece factory system every car seems to have lately ?
trouble is if its blocked exhaust prob it should dob it at all revs and be underpowered
It's definitely lost power!, Emissions are fine so expect it wouldn't be the cat? Car was serviced by a main dealer last week & they said nothing was wrong.
I may take off the back box & give it a try then.
Thanks
