Question

2007 Nissan Frontier 4 Cyl with engine P0420. Is there any way to know which sensor, front or rear (or is there just one?)

Answer 1

Unfortunately for the P0420 code the problem is more than likely the Catalytic converter. Instead of replacing the sensors, you could try using high-octane gas for a couple of weeks.

P0420 Nissan Code - Catalyst System Efficiency Below Threshold Bank 1
Read more: https://www.autocodes.com/p0420_nissan.html