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I’m sure I’m asking a question that is already out there but I’m not finding it. I have an 05 base model California exhaust. I have a P0420 code that I’m going to replace the O2 sensors first before I replace the catalytic coverts to see if that corrects the CEL. Under the car I have the 2 up steam sensors follow the exhaust back to the first catalytic converters (both sides) towards the back on top is O2 sensor, I’m thinking this is a downstream sensor then the exhaust pipes go back and meet at the “main” catalytic converter which at the back on top is another sensor. So if I’m right I have 2 upstream sensors(air/fuel) and 3 downstream O2 sensors.
 

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2012 OB , 2017 Impreza
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3,657 Posts
Personally, I would not waste money replacing O2 sensors if they are reporting 'normally' to the computer. HOWEVER: It is pretty common for the catalyst to 'wear out' after many miles of driving so it may need to be replaced.

Lets not forget that the FRONT O2 sensors are used to 'tune' the engine whilst it is running. The REAR sensors are mainly used to validate the function of the catalyst.

This link may be of intrest to you ==> https://allwheeldriveauto.com/subaru-check-engine-light-and-code-p0420-explained/
 

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2005 OBXT Limited, VF37, STI intake, 5MT
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Correct, your Cali model has 5 total O2 sensors (2 up, 3 down).

To support @brucep point, I'd do some diag and/or data logging first. $200+ on O2 sensors is an expensive gamble with no other evidence to support when it could just be a bad/dead cat(s).
 
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