Hi everyone. So for the last couple of month I’ve been having a problem with trouble code p0420. The car drives fine and have just had new timing chain fitted but still does it. The problem is that if I really give it some power trough the gears it will sometimes misfire at high rpm usually around 5000rpm like the power cuts. If you let off the power and put straight back on it does the same misfire and won’t go no more speed wise. I know the p0420 is related to the catalytic and I did fit a sports car not long before it started happening so would anyone be able to say if they think it’s the cat failing already or something else? All oxygen sensors are ok but downstream comes up with below limits coherence sometimes don’t know if that’s linked to the cat been blocked maybe?