P0420 $07e8 generic on 2011 Ford Fiesta se

Replace the catalytic converter. Hope that helps! Jim

I got it. I changed both sensors. The upper one badly bunt.

Changed both sensors twice and catalytic converter twice. Ford dealer reset the cpu twice and the car still pitches a P0420 code about once a week. Getting expensive.

Hi Leroy, something else is going on then. The P0420 code sets if the computer sees too much fluctuation in the signal from the downstream oxygen sensor. While you're driving the computer tests the catalytic converter by enriching the air to fuel mixture slightly. Then the computer looks to make sure the signal from the downstream oxygen sensor stays pretty much a flatline. This shows that the catalytic converter has neutralized the excess emissions from the test. But if there's a spike in the signal when the test is run the catalytic converter is not working. An exhaust leak between the catalytic converter and the downstream oxygen sensor, faulty wiring going to the downstream oxygen sensor, or the wrong catalytic converter can also cause a P0420. Your Ford Dealer needs to do a live data scan to see exactly what's going on. Hope that helps! Jim

Before replacing Catalytic Converter or O2 Sensors make sure to Check: the Operating voltage range and Internal circuit integrity (Open/Shorted) on those two O2 Sensors by OEM specifications before replacing them- then the issue could be at the internal Air/Fuel mixture, Air Injector Solenoid and firing in Engine or between PCM-O2 sensor(s) electric wiring, connections and ground. Thanks.