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2004 Subaru Forester XT
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Discussion Starter · #1 ·
Hey guys I just picked up my 04 Forester a few weeks ago as my first car and I love the car so much already but I recently just had my Check Engine Light pop up. Ran the code and got " P0420 - Catalyst System Efficiency Below Threshold Bank 1". I did some research and I'm pretty positive one of my o2 sensors is bad but I'm not sure which one. I've seen a lot of people saying the rear is the one that makes the CEL come on but then why would it say Bank 1 and not Bank 2? Anyways any help would be appreciated and if anyone knows exactly which one is which can you please let me know the location too because i'll be replacing it myself.
Thanks.
 
2017 2.5i Fozy CVT
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1,283 Posts
I would look into testing changing the first one with OEM but be prepared to buy a CAT CON, too.

Bad coil. spark plugs, upstream O sensor can lead to excess fuel being burned in the cat( not a good thing)...Spark plugs are how old?
 

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2004 Subaru Forester XT
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8 Posts
Discussion Starter · #3 ·
Spark Plugs were replaced at about 115k miles. Now at 150k.

By first one do you mean the one after the Cat or before the Cat?
 

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2004 Subaru Forester XT
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8 Posts
Discussion Starter · #7 ·
Just thought I’d post my results here. Replaced the o2 sensor before the cat by getting thru behind the passenger side wheel and fender liner after taking it off. Install was easy enough as it was my first ever fix for the car and she runs perfectly! Even better than when I bought her to be honest. More responsive, better mpg, and all power is back. Thanks for the help!
 
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