Hi, when you have a hard code you can not reset this, you could try a deep reset, than
you must disconnect the plus and minuc battery cables for a while ,after this connect
the cables and see if it is cleared.the 420 code means your catalytic converter does not
perform like it should or is on low efficiency, check also for a dirty o2 sensor

To increase cat efficiency a bit, make sure you have no misfire counts in Mode 6 in OBD mode. O2 sensor all show activity, Your filters are clean, oil is clean and fuel trims and fuel pressure look good with no plugged pcv valve, or collapsed pcv hose as a good starting point before condemning any cat converter as bad.