keep giving the same code

The Bank 1 should be the passenger side (cyl. 1,2,3,4) but give us the 8th digit of the VIN#, so I can tell you for sure. The P0420 is a catalytic converter efficiency code, which measured by the rear O2 sensor. Did you replaced the front or the rear O2 sensor? The P0420 does not necessarily mean a bad O2 sensor , it could be a bad cat. converter. I'd suggest to take it to a repair shop -who knows how to diagnose cat. efficiency DTCs - for a diagnostic because you might replacing good parts and wasting your $$. Zee

For a Ford, cat efficiency codes are usually dead on for Converter failures. FYI. Sorry about the news. Good luck.

You said you replaced co2 already. Two questions for you did you erased the code after replacing the co2? if not you need to erased the code if it keeps coming back, then you move to the next step which is to replaced the co1( oxygen sensor #1) obviously is the one you haven't replaced yet. It wont be a waste of money because they are probably bad anyways. If the problem persist then it could be the cat. I have an f-150 with the same issue I replaced both sensors and its working like charm now.