Honda code

P0420 on a Honda

P0420

In plain words: The catalytic converter on bank 1 is not cleaning the exhaust as well as it should.
Standard name: Catalyst System Efficiency Below Threshold (Bank 1)
System: Catalytic converter and oxygen sensors

General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.

What the computer saw

P0420 means the same thing on a Honda as on any car sold in the US since 1996: the catalytic converter on bank 1 tested below its threshold. Honda uses the standard generic code, so the meaning does not change by model.

Bank 1 on a Honda

Four-cylinder Hondas, such as most Civics, Accords and CR-Vs, have one row of cylinders, so they have only bank 1. That is why they show P0420 and never P0430.

Honda V6 engines, such as those in some Accords, Pilots and Odysseys, have two banks. On those, P0420 points to bank 1, the side with cylinder 1, and P0430 to the other.

Likely causes, most common first

A worn catalytic converter. The most common cause at high mileage.
A faulty rear oxygen sensor. The sensor after the converter judges it; a bad one fails a good converter.
An exhaust leak before the rear sensor. Outside air upsets the reading.
An engine problem that harmed the converter. Misfires or burning oil. Fix them before a new converter goes in.

What you can note yourself

Read every stored code. A misfire or oxygen sensor code beside P0420 changes the order of what to check.
Note your engine: four-cylinder or V6. It tells you how many converters the car has.

What to tell the shop

All the codes and your engine size.
Ask whether they checked the sensor readings and looked for exhaust leaks before quoting a converter.

This page gives no verdict on whether a car is safe to drive; a mechanic can.