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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?
 

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Did you consider that your catalytic converter is bad? You mentioned temperature; should be about 100 degrees Fahrenheit difference between the inlet and outlet (hotter) when the car is at operating temperature. I think this is what you were saying but just in case.

Is it possible you have an exhaust leak between the manifold and the catalytic converter? This will cause a P0420.

Regarding the upstream O2 sensor voltage; perhaps your connector or wire harness has an issue. You can you use a new wire to bypass the original to test.
 

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my 14 se catalyst grenade itself around 70k miles. might worth being replaced. in the 45k miles I owned my car only things that ive replaced on this car will be the axles, right front wheel bearing engine mounts, and a clutch(knackered release bearing) and the catalyst.
 
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