Mazda 626 1999 - Persistent P0421 and P0171 Codes After EGR Cleaning and Vacuum Leak Repair

Hi everyone, I own a 1999 Mazda 626 and recently encountered the OBD2 diagnostic code P0171 (Rich Fuel Mix - Bank 1). I took it to Sears Auto Shop, where they performed several checks: replaced the air filter and oil filter, and cleaned the EGR valve. However, after cleaning the EGR valve, two new codes appeared: P0141 (O2 Sensor Bank 1 Sensor 2 - Post-Cat) and P0421 (DTC for Catalytic Converter Efficiency Below Threshold - Bank 1). I replaced the O2 sensor as per the P0141 diagnosis. Despite this, both P0171 and P0421 remained. Eventually, a vacuum leak was identified in the intake manifold gasket and repaired. After fixing the vacuum leak, the P0171 code cleared, but the P0421 code persisted and reappears every time I reset the system. The car now exhibits intermittent stalling or hesitation under load. The P0421 code returns consistently after resets. I'm seeking advice on how to resolve this issue permanently — any insights into possible causes or next steps would be greatly appreciated, especially for a 1999 Mazda 626 with this specific combination of codes.