P0420 nightmare

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Apr 2, 2023
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Hey guys, I would really appreciate any insight.
Walmart in Indiana drained the transmission and double filled the oil in my wife’s ‘18 forester (I voted against going to Walmart)
It was towed to Subaru dealer and transmission was given a clean bill of health. However, moving forward two months a DTC P0420 popped up. O2 sensor on live data did not appear to be switching correctly, basically just constant and only changing reading with rev but no oscillation. Changed the O2 sensors, no change. Had a local “Subaru Guru” weld me in a new cat, no change, replaced leaking exhaust manifold gaskets, no change. Replaced spark plugs and found oil on them, no change. I have cleared the stored code after each one of these fixes. Permanent code remains and the P0420 inevitably pops back up. The O2 sensor data still appears to just be flat, only fluctuating in conjunction with throttle.
Walmart wants it taken to a shop, but the local place in WV refuses to do any diagnostic. When I brought it to them when the code popped up I gave them the DTC and the data I had pulled myself. Their response was, you need a new catalyst…$4100.” When I asked how they had confirmed that it was revealed that they had simply pulled the code and I was directly told that they did not look at ANY diagnostic data. Feel like I’m pretty much on my own and looking for any help I can get.
 

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