Ok I Need some direction here... I have a 1999 Mazda Millenia The following codes were read at autozone P0431 Catalyst Efficiency low bank 2 / P0421 Catalyst Efficiency low Bank 1. I think this might be the oxygen sensors. I changed out Bank 2 and 4 last year. Now I think it is 1 and 3. Does anybody know how to tell on this engine what bank is what? A diagram? Also do this engine have to have matching O2 sensors. I went to the bosch website at it gave me 4 sensors 2 before and 2 after. I bought the 13581 (bank 4) and 13572(Bank 2) and replace. So I am guessing I need 13574 and 13576???

13572 Oxygen Sensor - OE Type Before Catalyst O2 sensor located in front manifold/exhaust pipe.; Check / Replace Interval: 100,000

13574 Oxygen Sensor - OE Type After Catalyst O2 sensor located in front manifold/exhaust pipe.; Check / Replace Interval: 100,000 Miles

13576 Oxygen Sensor - OE Type Before Catalyst O2 sensor located in rear manifold/exhaust pipe.; Check / Replace Interval: 100,000 Miles

13581 Oxygen Sensor - OE Type After Catalyst O2 sensor located in rear manifold/exhaust pipe.; Check / Replace Interval: 100,000 Miles

 

bank 1(right bank) is closest to the firewall. bank 2(left bank) is closest to the radiator. your problem has nothing to do with oxygen sensors. changing them will do nothing, except cost you more money. you need to replace the pre cats. do a search, there are several posts about this. there is also a lot of ignorance about this, so be careful. those that tell you to replace the oxygen sensors are wrong.