Have 2012 Focus Titanium, threw check engine light code P0420. Checked the cat converter temp, it’s ok. The O2 sensor after cat shows stable voltage between 0.5-0.7 volts. The upstream sensor showed 0volts. So I replaced the upstream sensor and erased the code. Warmed the car up to operation temperature, and the new sensor still shows 0 volts, and no response to throttle. After driving for about an hour same code came back. What could it be?