P0420

Catalyst System Efficiency Below Threshold (Bank 1)

What does this mean? Well it means that the catalytic converter isn't working the way it should, or at least appears that way to the computer.

The cause is usually a contaminated or blocked cat. To see a list of what causes cats to die, see the Catalytic Converter page. In order to fix it properly and not kill your new cat, you would need to find and fix the root cause(s) first.

One way to get rid of the MIL for testing or off-road purposes, is to put a spacer in between the rear o2 sensor and the bung. This effectively moves the sensor slightly out of the exhaust stream so that it shows less fluctuation, exactly what the computer is looking for. A trip to your LAPS and ~$6.50 will get you a set of 2 'spark plug anti-fowlers'. Dorman Help 42009 http://amzn.to/2p4VsdD

These are what you are looking for, they are in the help section.

The inside diameter won't fit the o2 sensor, so you have to make the hole bigger with a 1/2in drill bit, or equivalent.

You can use a special o2 socket to remove the rear o2 sensor, or if you already have a box end wrench to fit you can just trim the connector a bit so that the box end will slide over it. Even after trimming the connector still latches closed when connected.