PO430 DTC in 1996 Mercury Sable: Causes and Solutions for Catalyst System Efficiency Below Threshold

This is my first post, and I'm hoping someone can help. My mother has a 1996 Mercury Sable that she brought in recently because it started missing and surging while driving on the highway. As soon as I checked the engine, I immediately noticed a vacuum leak at the PCV valve. After replacing all related components, I used my Innova 3100C OBD-II code reader to scan for trouble codes. The system detected a PO430 DTC: 'Catalyst System Efficiency Below Threshold (Bank 2)'. I consulted the Haynes repair manual and online resources, which led me to suspect a faulty oxygen sensor on the passenger side downstream of the catalytic converter. I replaced that sensor and cleared the code, but the check engine light (MIL) remained on. Before replacing the O2 sensor, I tested it with a multimeter and confirmed it was functioning properly — so it's not the root cause. I also considered whether the vacuum leak observed earlier might have triggered the PO430 code. My mother mentioned that the MIL had been on for a long time before she noticed the symptoms, which makes me wonder if the missing and surging were actually related to the vacuum leak rather than the catalytic converter issue. Another observation: when I hold the RPMs at 2500 and slightly press the throttle, the engine sounds like it's hitting a rev limiter — this is not normal for a 1996 Mercury Sable. This behavior suggests the catalytic converter might be clogged or malfunctioning. Given these symptoms, I'm looking for expert insight on what could be causing the PO430 DTC in this vehicle. Any suggestions would be greatly appreciated.