either cat or lambda
ppl say that sports cat throw up rear 02 faults
i replaced both the front and rear sensor and havent had a rear o2 sensor fault for 2 months now 🙂
if it is p0420 code then it more than the flow of emissions going past the rear sensor [i.e sports cat], it is down to how the ECM determines the responses of both sensors at any one given time and averages, the 2.2 ECM also does diagnostic checks every few hundred miles on both lambda's to ensure your CAT is not being fried, hence why o2 simulators do not solve the problems of rear lambda faults
funnily enough i used a genuine front lambda sensor and a NGK rear lambda to stop the p0420 codes