P0420 Fault After Installing Catco Converter on 2001 Camry LE - Causes and Solutions

I own a 2001 Toyota Camry LE 4-cylinder, non-CA model with automatic transmission, having maintained it through dealer service for over 10 years and reaching 142,000 miles. The vehicle has only had two Check Engine Light (CEL) events in its history—both related to the EGR valve. Recently, I replaced the flex pipe due to a severe leak. A local shop was unable to weld it, so I installed a complete new flex pipe and converter using Catco part #4061. This replacement was likely unnecessary, as I had previously kept the original Toyota converter intact. Fifty miles after installation, the CEL activated. Prior to this, there had been no CEL issues. The diagnostic tool revealed code P0420—catalyst system efficiency below threshold. A state emissions tailpipe test (conducted after resetting the CEL) passed successfully. The local mechanic suggests a faulty front catalyst converter, but I find the timing suspicious. If the front converter were failing, wouldn't it have failed during the emissions test? The P0420 code is now the only one present—CEL reactivates after two cold starts and 30 miles of driving, even after disconnecting the battery. Could weak or faulty front air-fuel (AF) or rear oxygen (O2) sensors trigger a P0420 code despite using a new aftermarket converter? Is it possible that this Catco unit meets emissions standards but fails to meet the Camry's ECM performance expectations? What should I do first to resolve this issue? Should I continue driving the vehicle as-is and have AutoZone check for any new codes monthly, or is there a more effective diagnostic step?