Q1: Where is the Sensor located?
Q2: How to replace it?

 

Hello,

P0430 - Catalyst System Efficiency Below Threshold. It means that it may not be a sensor, but rather a catalyst or exhaust itself.. Is there any unusual smell coming from the car? Any exhaust leaks?
I saw some catalyst cleaners in some parts stores, but I have no idea how efficient or safe to use they are, so I wouldn't trust those.. Hopefully someone else could chime in.

Hope this helps and best of luck!