P0420/p0430 是一回事，四缸车就是420， 六缸则可能两个都有

建议去这里看看，http://hubpages.com/autos/Catalytic-Converters-For-Dummies

 

Here is a huge secret that has not made it into the public domain yet:

If you remove a rear oxygen sensor and pack it with red RTV silicone, that will destroy the oxygen sensor in a way that will trick your car's computer into thinking the catalyst is okay. The sensor will not work well enough to report that extra oxygen is coming out of the converter, but it will still generate enough voltage to pass the car's oxygen sensor activity test. Never do this to a front oxygen sensor because it will mess up the fuel system.

其实就是死马当活马医。

一般来说，下游的o2 sensor 是汇报Cat 状态的。先拿硅胶蒙上试试，不行再换新的。

尽量不要换cat, 太贵了。