P0420 BMW -what is causing this

Question

The vehicle is a BMW M3 with P0420 code, service engine soon light is on!

To clean the MfA sensor is relatively easy!
To change o2 sensors, this first is located where.
Thanks

Answer 1

P0420 Catalytic below thresold Bank 1

What causing P0420 is the catalytic is below its efficiency, Changing the front Bank 1 Sensor 02 sensor may not resolve the issue.

Here the location for the front 02 sensor, the catalytic convertor are one peice with the exhuast manifold.