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2011 Subaru Forester X Premium (NA)
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Discussion Starter · #1 ·
Finally made an account so this is my first post. Sorry for any mistakes. I have a 2011 Forester X Premium and I had wanted to replace or remove the second cat in the line. Because I still have the first one wouldent I be ok to remove it? Also I have heard of a code that would make it unable to pass inspection but would I be able to just put the old one back on for the inspection? Finally would I see any performance or airflow increase with this? Thanks in advance for ur time guys.
-N

P.S. I have been into cars my whole life but started only recently getting into the tech aspects of them. So I hope to learn much more about them!
 

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2010 X Limited, 2.5L NA, 4AT. Purchased as the second owner in 2020 with ~126K miles.
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It is highly unlikely you will see a performance improvement.

It may throw a code on you because you change air flow, back pressure. Maybe fuel mixture. Also there are calculations performed that compare front O2 sensor readings with the downstream O2 sensor readings.

Do you think this one is plugged?

Mine has gotten holes in it so I was going to replace it but I didn't want to spend the money so I patched it. I would rather wait and change the front and rear cats at the same time. Of course there is the extra cost of the O2 sensors.
 

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2011 Subaru Forester X Premium (NA)
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Discussion Starter · #3 ·
Do you think this one is plugged?
Im actually not sure. Tbh I dont think so but I want to run an aftermarket exhaust (Not super loud just a better sound. Dont want to be THAT guy yk?) so I figured I would start with this. I also didn't know there was a downstream O2 sensor. Where is that located? Thx for the reply tho man!
 

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Super Moderator
2010 X Limited, 2.5L NA, 4AT. Purchased as the second owner in 2020 with ~126K miles.
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1,550 Posts
The objective is better sound? Others are more informed on that than me.

Here is your system from the Subaru site. O2 sensor before and after the first catalytic converter. I think if you change exhaust after the flange, you'll have the best of both worlds. The N/A just can't be pushed into better performance.
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2007 Forester Sports XT 4EAT
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@nsomniac the rear CAT is not monitored by the ECM... rear O2 sensor. I believe it was added for higher emission standard States (Californian/New York) that use a "tailpipe sniffer" as part of the emission testing procedure.

If your State doesn't use a tail pipe sniffer as part of the emission testing, I see no reason why you couldn't delete the rear CAT. Keep in mind that CAT is worth some serious money if you recycle it!

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 
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2011 Subaru Forester X Premium (NA)
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Discussion Starter · #7 ·
@2.5x_sleeper Cool! Ill defiantly think about that. I also saw on your description that you had an '03 turbo. U turboed the stock motor right? Do u have a post summarizing what you did? And do you think its possible with an '11? Thanks for ur input and sorry for all the questions lol.

-N
 

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2007 Forester Sports XT 4EAT
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2010 Forester 4EAT
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The exhaust manifold/converter on my 2009 Outback disintegrated... and all the shrapnel clogged the rear converter!
I replaced the manifold with a cheap replacement, and deleted the rear converter. Actually sold it for good money. lol
The O2 sensors are ahead and behind the front converter. The front O2 monitors the air/fuel to adjust engine parameters, and the rear tells how well the converter is working. Neither sensor is tied to the rear converter in any way. If it goes missing, the computer has no way of knowing. My car has been this way for 30k miles... and no engine light. I cut out the converter and spanned the distance with a woven stainless flex piece from Advance Auto. I have tools and a welder, so it wasn't that much of an issue. An adapter on each end was used to attach it to the existing pipe, which was different sizes. Good Luck!
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