04 Saturn Vue 2.2 changed 02 sensor bank 1 sensor 2 now getting code PO420??

The diagnostic trouble code (DTC) P0420 "Catalyst System Efficiency Below Threshold" is one possible outcome following the Catalytic Converter Monitor (test). This monitor is one of the last tests to be performed by the powertrain control module (PCM) because other failures in the system could compromise this test's result. Catalyst efficiency is tested by observing the oxygen sensor signals both before and after the catalyst. Before the catalyst, the signal should appear as a sine wave as it switches back and forth between rich (0.9 volts) and lean (0.1 volts). After the catalyst, the signal should be flat-lined on the rich side (about 0.7 volts). This is because a healthy catalyst will store oxygen and therefore remove the "lean" portion of the signal/waveform. If the signal after the catalyst still appears as a sine wave, the PCM concludes that the catalyst can no longer store oxygen and should therefore be replaced. In your specific scenario where a downstream (post-cat) O2 sensor was just replaced, I would theorize that this sensor had failed and was flat-lined. Then when it was replaced two things happened: 1.) the oxygen sensor began to work properly which allowed the oxygen sensor monitor (test) to complete, and 2.) the catalyst monitor then ran and used the new (accurate) sensor data to determine that the catalyst efficiency was below threshold. You can read more here: https://www.yourmechanic.com/article/p0420-obd-ii-trouble-code-catalyst-system-efficiency-below-threshold-bank-1-by-jamahl-walker