What does that mean?
Basically this means that the oxygen sensor downstream (after) the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.
Symptoms
You will likely not notice any drivability problems, although there may be symptoms.
Causes
A code P0420 may mean that one or more of the following has happened:
[ul][*]Leaded fuel was used where unleaded was called for[*]An oxygen sensor is not reading (functioning) properly[*]The engine coolant temperature sensor is not working properly[*]Damaged exhaust manifold / catalytic converter / exhaust pipe[*]Retarded spark timing [/ul]
Possible Solutions
The simplest thing to do is to reset the code and see if it comes back.
Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.
One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.