1st of all thanks to everybody for your help in the past and happy holidays. Now then I've got an error code 420 catt eff 1. I swapped the rear o2 sensors, cleared the error code and await the return of the same code before I bring my car into the dealer for a new catt.(my car is a 2k w/less than 80k). I'm wondering though if I should also swap the front o2s.