Low Voltage O2 Sensor

[Wapiennik]

1996 328i 5-speed

I replaced all four O2 sensors and my catalytic converters with aftermarket ones almost five thousand miles ago, and my cars been running great. One day driving to school my check engine light comes on. I plug in my scanner and it's P0430 (Catalyst Low Bank 2).

I monitored my O2 sensors with the RPM at 1,500 and noticed my scanner is reading a low voltage on one of the O2 sensors. With the catalytic converters and O2 sensors being new, would this be more suspected of a faulty O2 sensor, or a defective catalytic converter?

Any advice would be appreciated. Go figure this happens one month before my inspection is due



Also, please confirm my understanding but O2S22 is referring to Oxygen Sensor 2 (post-cat?) on bank 2 (cylinders 4-6?) which would be the rear three?