Would get rid of the P0420.

To add or replace the O2 sensor, jack the side of the car on passenger side up about 12 " and secure it on car stand. That would give you enough room to get under the car and access the O2 sensor on the pipe.

The pipe is extremely hot and sensor may be rusted or stuck. You may have to spray some liquid wrench on the threads and wait to loosen it. When putting it back, apply some antiseize high temp grease on the thread so it will be easy to tak eout next time.