Hi Scotty I have a 2010 Ford focus and I have an ongoing issue with the po420 code . The catalytic converter has been changed but engine light won't stay off for the same code . The oxygen sensors are working fine as well. What else could be the problem?

that is a complex computer driven feedback system that measures exhaust gas concentrations to set the proper air fuel ratio on the fuel injection system. TONS of things can do that, from bad injectors, weak fuel pump, bad sensors (theyare VERY hard to test fully without a pro and a freeze frame oscilliscope) BUT , if it runs fine, I've seen guys ignore that for YEARS with no harm;