PO420 and PO135 Fault Codes on 2002 Honda Accord (PRIZM) - Diagnosis and Repair Help

Hello everyone, I'm looking for help with a recurring issue on my wife's 2002 Honda PRIZM. The car started showing the Check Engine Light (CEL), and after having the codes read, we found both PO420 and PO135. The vehicle has around 90,000 miles and is driven daily through busy NYC streets — essentially treated like a race car. Initially, only PO420 was present. Given the mileage and usage, I replaced both O2 sensors and the catalytic converter, assuming it was time for maintenance. After clearing the code, I let the engine run for about an hour before shutting it off. Later that day, when I tried to start the car again, the CEL came back on. I took it to a scanner and now both PO420 and PO135 are present. I'm at a loss as to what's causing this. Any insights or suggestions would be greatly appreciated — especially for someone with limited experience in diagnosing these codes on older Honda models.