where is the sensor for P0421 error code

Question

according to CarMD its a Oxygen sensor that is malfunctioning, but I don't know which one it is.

Answer 1

P0421 KIA - Catalyst Efficiency Below Threshold Bank 1

Read more: https://www.autocodes.com/p0421_kia.html

The code mean that wither the O2 sensor or the catalytic converter is not working properly. Usually the problem is catalytic converter.