P0420 and P0135 Diagnostic for 2003 Jeep Wrangler 4.0L Six-Cylinder

My 2003 Jeep Wrangler with a 4.0L six-cylinder engine has recently triggered two OBD-II diagnostic codes: P0135 and P0420. The vehicle runs smoothly with no noticeable performance issues, and I've only driven approximately 29,000 miles. After clearing both codes initially, the P0420 code reappeared after four days. The vehicle is stock except for a K&N air filter and a PaceSetter cat-back exhaust system, both installed two months prior to the fault appearing. There are no visible exhaust leaks, so I'm unsure whether the issue stems from engine performance, emissions components, or a sensor malfunction. Given that fuel injection cleaner was previously used, I suspect it may have contributed to injector buildup—though this remains unconfirmed. I'm seeking advice on how to properly diagnose and resolve the P0420 code in this specific model.