P0420 Fault on 2002 Nissan Frontier XE 4-Cylinder 2WD Manual Transmission - After Catalytic Converter and O2 Sensor Replacement

The P0420 diagnostic trouble code has persisted since my catalytic converter, both oxygen sensors, and spark plugs were replaced approximately 5 months ago (about 5,000 miles). The vehicle runs normally but exhibits a noticeable power loss during acceleration. Additionally, the engine becomes louder than usual around 2,500 RPM, and I frequently hear a rattling noise from the rear driver's side underbody. All other components including the gas cap and engine coolant temperature sensor were replaced last summer during a radiator replacement. Despite these repairs, the P0420 code remains active. Any insights into potential causes or next steps would be greatly appreciated.