The issue is that the rear O2 sensor is before the three way cat. The warm up cat in the Sentra was the primary cat with the front O2 sensor near the head and the rear at the bottom after the warm up cat. The two sensors have to have a cat between them or they are not going to work properly.
The rear sensor only checks the efficiency of the cat, so without a cat between them, it doesn’t work. Someone may have done something to trick the computer into thinking the rear O2 sensor is working.