2000 Chevrolet S 10 Got Po420 Code 9k On New Gm Crate Engine Last Owner Did

I got a po420 code. 9k on a new GM crate engine.(The last owner did not believe in proper service)New cat(Walker).New 02 sensors(Bosch).I checked for leaks(vacuum and exhaust).None found.I cleared the code and have rechecked for active or pending codes every 2 days for over a month.None found.MPG and performance are great.Could the aftermarket cat and/or 02 sensors have anything to do with it?(false code?)Or is it just a fluke? Thanx Shep. (Retired tech trained on asian imports)