What diagnostic steps fix persistent P0420 after new O2 sensors on 2015 Escape?

[Richard.G]

I'm a fairly competent DIY mechanic, but the check engine light diagnosis on my 2015 Ford Escape has me stumped; the code is P0420, indicating a catalyst system efficiency below threshold, but I've already replaced both upstream and downstream oxygen sensors with OEM parts, and the light came back after about 50 miles. There are no exhaust leaks I can find, and the fuel trim numbers look normal when I monitor them with my scanner, so I'm hesitant to just throw a costly catalytic converter at the problem without being sure. For mechanics who deal with persistent P0420 codes, what's your systematic diagnostic process to rule out other causes? Should I be looking more closely at potential vacuum leaks, faulty fuel injectors, or even a lazy thermostat causing the engine to run too cool, and what specific live data parameters are most telling when trying to confirm a truly failed cat versus a misleading sensor or other underlying issue?