I’m sure I’m asking a question that is already out there but I’m not finding it. I have an 05 base model California exhaust. I have a P0420 code that I’m going to replace the O2 sensors first before I replace the catalytic coverts to see if that corrects the CEL. Under the car I have the 2 up steam sensors follow the exhaust back to the first catalytic converters (both sides) towards the back on top is O2 sensor, I’m thinking this is a downstream sensor then the exhaust pipes go back and meet at the “main” catalytic converter which at the back on top is another sensor. So if I’m right I have 2 upstream sensors(air/fuel) and 3 downstream O2 sensors.