I have a 2001 Montero Sport which I am trying to get smogged. The dash light is on and the tester said it was a code po421, warm up catalyst efficiency below threshold (bank 1). I live in California and they are strict. How can I find out if it is my oxygen sensor? It runs great.

 

It is NOT the oxygen sensor. You have a bad (worn out) catalytic converter. The fact that PCM had picked up the P0421 code means that ALL O2 sensors are working, just not getting the readings that will correspond to the proper function of the catalyst. Look up few post back. There are people dealing with the same issues given the similar age of these vehicles.