If you are removing you catalytic converter on your Mini and want to prevent the post cat O2 sensor from causing a P0420 code this is a simple trick you can do. First I know you would only remove a cat on a track car that would never be used on the street and most of all, not in California where they would lock you away for life.
What you need
Drill and 7/16 drill bit
2 18mm Spark plug Non-Foulers (they come in a two pack)
Go to your favorite parts store and buy two pack of Spark plug Non-Foulers I got mine at O'Reilly for $5.99
Detroit Tuned has a O2 extender that does the same thing for $21.95, if you prefer
Once you have your Non-Foulers you will need to drill one out so the O2 sensor can fit into it. A 7/16 drill bit is the size you will want to use.
This picture shows how the O2 sensor fits after it has been drilled out.
Now screw the two Non-Foulers together and than into the pipe.
Then screw the O2 sensor into the non-foulers
Yes I know that is a cat, this is just to show what is will look like when finished.
If you have unplugged the O2 sensor plug it in and your done!
They're are 3 other ways you can avoid getting a P0420 code. One is if you are getting a custom tune the tuner can code it out. The other way is to put a resistor inline with the O2 sensor. I have been told you can buy dummy O2 sensors but could not find them. This took about 15 minutes and was the whopping price of $5.99.
Now go have fun!