A P0420 code is a trouble code on OBD-II equipped vehicles. All vehicles from 1996-newer sold in North America support this generic powertrain code. The code's description is Catalyst System Efficiency Below Threshold (Bank 1). OK, now tell me that again in plain English please.... Let's break it down: The catalyst system being referred to is your three-way catalytic converter; so the catalytic converter is not working properly (i.e. It's not efficient). Bank 1 refers to the side of the engine which contains cylinder #1. On inline engines such as 4 & 5 cylinder engines, there is only one bank. So, to really simply things let's just say that the efficiency of the catalytic converter and/or exhaust pipe and/or O2 sensor is not so great.