okay the TWC is the catalytic converter (Two way converter).
the P0420 is the bank 1 and the P0430 is bank 2 .
Bank 1 being the side with the #1 cylinder
There are 4 sensors two upstream A/F sensors and two down stream o2 sensors ,
The once marked in a red box are the downstream sensor after the TWC, these are the sensors that monitor the function of the TWC, if they are functioning below the given set monitor specs, then the code will trip to let you know if the TWC are not working correctly .
If the sensors are good and there is no exhaust leak and you don't have any misfire codes or any other codes, then the chance that the TWC is bad is very likely.
The cost of the sensors very from dealer to dealer , you can get original Denso sensors at Rockauto.com at a very good low price .
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