2000 Chevrolet Astro 4.3L Engine PO420 Code: Diagnosis and O2 Sensor Testing Guide

My 2000 Chevrolet Astro has 92,500 miles on the odometer. The 'Service Engine Soon' light started coming on around 90,000 miles. Using an Actron scanner, I detected a PO420 code for Bank 1. However, the device also reports one of two codes without providing further details about the second code. I ran the engine until it reached operating temperature and then opened the electrical connector on the downstream oxygen (O2) sensor to test voltage. Initially unsure which wires to measure across, I tested all combinations but found no measurable voltage output. As a result, I replaced the O2 sensor. The 'Service Engine Soon' light returned shortly after, and the PO420 code reappeared. I cleared the code, but it returns again. The engine was tuned 9,000 miles ago. The dashboard temperature gauge reads normally, there is no visible exhaust system damage, and I don-'t believe the timing is electronically controlled. Should I be investigating an upstream O2 sensor before the catalytic converter? Could that be the second code? Could you please clarify which two wires to measure voltage across on the downstream O2 sensor? The connector has two black, one blue, and one white wire. Any guidance would be greatly appreciated.