Cheers, that makes sense. Wiring looks OK, just difficult even getting to the connection. I guess there's a tool for the rear O2? got one for the front but it's useless for the rear. So even after changing the O2 sensor I could still be having problems if the cat is blocked? After the oil change and O2 replacement and if it still runs badly, would removing the front O2 sensor remove enough back pressure to prove the cat is now blocked?