2015 Forester XT P0420

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Hello all,

First time posting here because I have no more options, the local Subaru shops cannot figure out what is wrong with our Forester. We have the P0420 code. I have replaced the catalytic converter, AF sensor, rear O2 sensor, PCV valve, Spark Plugs. All OEM parts, I tried 3rd party sensors but that did not work so I went back to OEM sensors.

The latest test drive at the local dealer showed that I have very odd rear o2 sensor readings, spikes in the data, sometimes shows consistent readings but then spikes. I was told I need to replace my catalytic converter, even though it's brand new now.

One odd thing is the engine code will go off most of the time when we press the brake pedal to slow down after cruising for awhile, or coming off cruise control. It's been very frustrating, so I'm hoping someone here can help. Spent about $4k on this issue with no improvement.
 
Check the harness for any rodent intrusion and or harness damage (short to Positive or Ground)
Also, if the exhaust has been repaired previously and "electric welded" there may be software damage to the PCM.
A PCM flash can be performed.
Check for any vacuum leaks ie: Intake manifold, brake booster, PCV valve, EGR including MAF performance.
Check for any exhaust restrictions as well ie: exhaust back pressure or exhaust leaks.
Find a better technician that can test all of the above noted.
 
Thanks for this list of possible issues. It'll help me move forward on this. No exhaust damage/weld work has been done.
 

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