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01 forester sport
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Discussion Starter #1
i bought my first subaru three days ago from a dealership in PA. ( 01 forester sport 5 speed. 187800 miles. )i drove it home and my check engine light came on. i was getting great gas mileage and then my mileage went down to about ten miles per gallon. i got home around 930 from the trip home , so the next day i went to autozone and had them read the code. it came up p0139. i think thats the code but im not exactly sure. it was the o2 sensor for the exhaust. it cost me a hundred bucks for a lifetime warranty. replaced that , and took off my positive lead for a half hour to reset everything. my gas mileage went back up again , but i was driving two hours ago and the check engine light came back on. i turned around and went back to autozone. p0240 -Catalytic Converter Efficiency Below Threshold came up. im not new to modding cars , but im new to subaru. so does this mean that the o2 sensors are bad on bank 1? or does this mean that i need to replace the cat?? any info would be grealty appreciated. thanks.

tony from rochester michigan.
 

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02 Forester L (sold) Manual
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1,220 Posts
Do NOT assume the cat needs replaced. P0420 is a diabolical code that IMO is best viewed with suspicion. It is only telling you that the rear O2 sensor is not getting a clean exhaust reading.

Since the front O2 sensor went bad and your mileage dramatically decreased, I'd guess the computer made the fuel run way rich. This has contaminated the converter with excess fuel, which may burn out over time. There's a chance it may have permanently damaged the converter also. But if it runs fine, has good power and the only thing amiss is the dreaded P0420 code, I'd absolutely do the $5 fix. I did it to ours. :)
 

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01 forester sport
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Discussion Starter #4
hmmm it was the rear o2 sensor on the exhaust that went bad. i called the dealership in PA and they said that the sensors go bad alot. but anyways whats this 5 dollar fix and how do you do it??? id rather spend five bucks then replace the other o2 sensors. or should i just replace them anyways?
 

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2007 Forester Sports XT 4EAT
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40,088 Posts
If the rear O2 sensor was bad, the $5 fix wouldn't have helped. :frown: Here a link to the thread. :smile:

Bobby...

My MODding Journal
 

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02 Forester L (sold) Manual
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The rear one may have also gone bad as a result of the front one giving false readings. I would replace the front one as a maintenance issue just in case. Around $90 I think for the part, but then you'll know that's not the problem. The car went big-rich for a reason, and the rear sensor does not control the mixture at all, it's just a 'reading' sensor.

P.S. Who did you buy the car from in PA?
 

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01 forester sport
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Discussion Starter #7
alright. might as well just to make sure like you said. where is the front o2 sensor located? is it on the exhaust also like the rear o2? and i bought it from oakhill motors in waterford , PA.
 
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