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Discussion Starter · #1 ·
I am trying to diagnose a cat with the classic P0420 code ('05 Outback 2.5L). I have read through all the forums and one question remains; on my scanner which O2 sensor reading is for the downstream sensor? I have all these O2 readings and cannot for the life of me figure out which is the actual O2 vs. the upstream Fuel/air sensors.

I know I am looking for a steady reading at roughly .5V but don't know which reading to look at.

I have the Ancel AD310 scanner if that matters.

Thanks for any input!
 

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2012 OB , 2017 Impreza
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I have all these O2 readings and cannot for the life of me figure out which is the actual O2 vs. the upstream Fuel/air sensors.
The AIRFLOW sensor is the MAF sensor. (Mass AirFlow) It usually measures in grams/second (g/s)
I am not sure what you mean by "FUEL" sensor...there may be "fuel pressure" sensor.
 

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I HIGHLY suggest you spend some time reviewing Utube videos from
Both of these guys are professionals who have several videos available showing how to diagnose underperforming catalytic converter using codereader to review PIDs. (realtime data)

Do not make the mistake of looking for Subaru videos.... The process of troubleshooting is essentially the same on ALL vehicles.
 
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