XXXXXXXXXXX,
P0420 sets when the PCM sees both the front and rear O2 sensors reading too closely for a determined amount of time which in turn tells the PCM that the cat is not functioning properly. When this code sets it con be one of a few things such as if a rear O2 has been replaced recently and the front O2 is still original or substantialy older. An exhaust leak close to either O2 sensor. A partially clogged cat that contains alot of soot. A slow or bad rear O2 sensor.
Check for any exhaust leaks and repair any that may be present. Using a scan tool, monitor the front and rear O2 sensors and confirm both are switching continuiously from rich to lean and that they are not reading around the same millivolts. If one is slow, replace it. If rear has been replaced recently or needs replacing due to inactivity be sure to replace the front also. If none of the above are at fault then the cat will need replaced.
You can try removing the cat on bank one and blow compressed air through it to remove excess soot build up and it may work for a while. I had done this to my winter beater car (96 Ford Taurus) and have not seen the code return in 2 years but it is no guarantee.
Shawn
ASE Certified Technician
PA Certified Inspection emissions, ASE Brakes Suspension, Lead diagnostic Tech.20 years exp