2000 vw jetta 2.0 4 cyl

I’m having a problem with a check engine light that indicates the fuel mixture is burning to lean / post O2 sensor reading.
I recently purchased this vehicle from a private seller whom I’m in contact with.
It’s a CARB emissions car, and he recently changed the preO2 sensor, but doesn’t know if he used a cali-emissions compliant sensor.
I’m wondering if the 02 sensor (pre) could be the cause of the fault. I don’t want to replace the cat-converter unnecessarily. … that’s very expensive guess work.

Any suggestions?