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2013 SE Automatic 110k miles - Z9 color ;)

About 3 months ago, my CEL came on, scanned and found code P0420. Code is centered on cat and O2 sensor issues. Ran a bottle of Cataclean through the tank and it cleared without having to replace the intake manifold and all for a new cat.

Fast forward to now, CEL is on again with new code P0054, relating to electrical resistance pointing to faulty wiring or downstream O2 sensor. Starting to lead me to believe the sensor is dying, and I was going to just hop under it and replace that. However, when looking up P0054, the top search results include this forum, indicating that this is maybe a common issue for Fiestas.

Wondering if replacing the downstream O2 sensor is a sensible first step, or if there’s critical information related to the vehicle I’m missing here. There’s an unfamiliar sound from the driver’s side of the engine bay as well, kind of a thin metal rattling above 15MPH that could point to other related issues, but could be it’s own thing. Figured I’d mention it.

Any advice is greatly appreciated, help me on my quest to drive her til the wheels fall off.
 

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Did you measure the voltage output of the O2 sensor? Finding out whether it's functioning properly would be the first step, not just throwing parts (and your money) at it.
 

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Rattling sounds like heat shield related or possibly exhaust hangers. I have the same code and I’m thinking it’s probably just the sensor
 
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