Question

2006 Nissan Sentra 1.8L, can the A/F ratio sensor be bench checked for serviceability? I got the engine codes P0420 and P1278

Answer 1

No. The 02 sensor is a airfuel ratio sensor which is a dual planar . Even if you can  bench testing is only a static test. The concepts of the OBD is the catalytic convertor, where everything for  engine management is built around the oxygen sensor, If the 02 sensor if it is  not working the catalytic is not going to work. When you set a slow 02 response , check the MAF sensor airduct from the throttle body leaks there would cause the 02 to go slow. A misfire would cause the 02 sensor to go fast.Vacuum leaks which are unaccounted air that get past the MAF sensor would  cause the 02 to go slow beside a higher RPM also if the engine oil is overfilled to a 1/2 quart more.

Answer 2

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

P1278 NISSAN Code - Air Fuel Ratio Sensor 1 Bank 1 Circuit Slow Response
Read more: https://www.autocodes.com/p1278_nissan.html