o2 sensor location

[blackmax23]

Ok, I have a P0420 code, catalytic inefficient, I have a hi-flo cat and it's making my o2 sensor go nuts, so I want to put in an o2 simulator, but I have no idea which o2 sensor it is. I always thought that for an o2 sensor to measure the efficiency of the cat, then the sensor would have to be located after the cat, towards the rear of the car. Typically, it would be in between the resonator and the cat. But, after looking under my 99 max (cali emissions), I see that all four o2 sensors are on the y-pipe. Which one is analyzing the cat?

[maxhopper]

The code is for the right bank pre-cat, which is the one closest to the radiator.

[blackmax23]

Thank you! Now here's the next question...How do I fix it? And do you think, since the code possibilities could be ignition timing malfunction, that my JET v-force module, which does modify the ignition curve and spark advance, could be the culprit?

[maxhopper]

You could just buy a y-pipe and eliminate the precats all together. I'm not really familiar with the parameters of the JET ECU, but if it modifies the fuel maps, you could be running too rich causing the catalyst to fail.

[blackmax23]

If it's saying my pre cats are inefficient, then wouldn't the y pipe, with no pre cats at all, still trip the cel?

[brianw]

No, since precat efficiency is not measured, it's the efficiency of the main cat that's measured under OBD-II parameters.