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Discussion Starter #1 (Edited)
My Fos turned 100,000 miles last month and I decided to replace my O2 sensor as my MPG has slowly declined per my records shown on my 100,000-mile MPG Curve thread. http://www.subaruforester.org/vbulletin/f107/my-100-000-mile-mpg-curve-65816/

I purchased a front O2 sensor from Jackie at Annapolis Subaru and I saved >$40 since the local dealer quoted a cost of penny's less that $200 for the part. Thanks Jackie! The part number is 22641AA360, and the description on the receipt states "SENSOR AY AI".

The sensor is located in the front of the engine compartment near the bottom, and can be viewed and reached from the top. Standing in front of the compartment, you can see the sensor when looking down between the radiator fan and the engine pulleys/belt. The sensor is located directly below the power steering pulley.

I used a crow's foot socket that I rented from AutoZone since there wasn't enough head room for spark-plug socket type tool. It was on tight and I could not loosen the sensor with just a ratchet. I ended up getting under the passenger's side wheel well and removing a few plastic screw-plugs that hold the plastic splash guard that needs removed to replace the fog lamp. The cover did not need removed, just bent enough to get in the engine compartment from there...it was easy. From there I used a breaker bar which did the trick. Don't get me wrong, there isn't much room and it wasn't easy to get everything in place, but once I did, the bar easily loosened the threads.

From there, it's simply unscrewing the sensor, removing the wiring harness, removing the clip that holds the orange-colored wiring sleeve, and installing the new sensor. Below is a photograph of the front O2 sensor and wiring harness taken while standing at the front passenger-side corner of the car, and it also shows another sensor located directly below the passenger-side spark plugs location. The additional sensor appears to also be a part of the exhaust, but I'm not sure what it's for?



If everything went smooth, the O2 sensor can be replaced within 1/2 hour, but it took me probably 1 hour all in all after figuring out how to loosen the darn thing.
Hope this info helps someone...some day.
 

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The Modfather
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There are two oxygen sensors on the exhaust system. One before the catalytic converter and one after. You are showing both of them. The one before helps determine the air fuel ratio. If it goes bad it will effect your performance and MPG's to varying degrees depending on how bad it is. I always like to change the front one around 70 - 80K miles on the ODO. No point in waiting till it gets really bad. It can be performing sub par and not throw a code. The rear sensor only tells the ECU how the cat is performing efficiency wise. It won't change the way the car operates. The rear one has been known to give bad info and need to be replaced. It can also be indicated as bad (the rear one) if you have an exhaust leak, it suddenly gets super cold if you get the area packed with snow or the cat is actually bad. For some reason it seems more often than not, the cat is OK and it's something else wrong. An EGT sensor is only on the Turbo Motor AFAIK. Here is a scan to show you the system:

Exhaust ? Illustration #3, Subaru FORESTER ? Subaru parts catalog
 

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Discussion Starter #4
Wow, the cat is located THERE??? I always thought it was the ballooned-portion of the exhaust pipe half-way between the muffler and manifold, shown located just to the left of the circled 4 on the schematic in your exhaust illustration, Peaty.

Ya learn something new everyday.
 

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Sorry to drag up an old thread, but I'm throwing a P0131 code and am going to take a stab at replacing the front o2 sensor. I had thought the rear o2 was right around where Buddur described, near the circled 4 in Peaty's linked diagram. Is it certain that the sensor labeled "Additional sensor" in the photo is the rear sensor, or more importantly, that it isn't the front A/F sensor? I want to be sure that the one with the orange wiring sleeve is indeed the front sensor. And out of curiosity, if that other sensor isn't the rear o2 - what the heck is it??

I have an AT X not an XS, not sure if that makes any difference here.
 

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I just replaced the 02 front sensor (same part number) on my 03X AT due to the check engine lite coming on and the CEL throwing a P0032 code........which indicates the front sensor....

And.......the check lite stays on and cannot be cleared and then stay off.......so, I'll be heading over to Bruce Bottomley's home next week to test the current reading at the plug-in for the sensor.....

Apparently, if I understand the info correctly, the front 02 sensor is purely electrical in nature and has to do with a heater for the sensor that warms it so the probe works properly. Therefore, I'm told that either the new sensor is flawed (Jackie is checking about this) OR there's a ground issue at work......in either case, I'll know more when Bruce helps me check the current readings....

I didn't think about the sensor, BTW, until the check lite came on......and I'm at 132,600 miles. BTW, my MPG is still pretty good so the sensor may not be affecting it.

Steve
 

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Sorry to drag up an old thread ...
That's never a sin here. It's strongly encouraged! It keeps related information together, making the search function produce fewer, more effective results.

I have an AT X not an XS, not sure if that makes any difference here.
I'd be really surprised if there were any fundamental difference between an X and an XS. Differences were in some of the big options (and some of the little ones as well).
 

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Just had a silly thought - given that space is at a premium and it's hard enough getting the O2 off, how does one go about setting the right torque when installing a new one? Do you all just tighten it until it feels snug? I couldn't fit a torque wrench in that space if I tried.
 

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I know this thread is more than 4 years old but here's my experience in replacing mine.

Issue: My 06 X AT runs fine when cold but starts misfiring all 4 cylinder when on a stop & go traffic or climbing up hill. Then it throw the CEL p0301, p0302, p0303 & p0304 but no o2 sensor errors.

research: after browsing the web for days, lead me to believe it might be my front o2 sensor.

fix: Denso front o2 sensor at amazon for about $100, a breaker bar, a long extension and a crow feet wrench.
Crow feet wrench into the the sensor > long extension > breaker bar > a quick turn on the bar wala! it came off without any drama.
Looking at my picture 2, I was able to remove it from the top of the engine without crawling, lifting the car, removing any cover or scraped knuckles.

So far after driving a week no missfires or CEL.


 

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Just had a silly thought - given that space is at a premium and it's hard enough getting the O2 off, how does one go about setting the right torque when installing a new one? Do you all just tighten it until it feels snug? I couldn't fit a torque wrench in that space if I tried.
An old thread but a useful one.

2007 Torque Specs N/A Engine (Subaru Service Manual-Exhaust Section-General-Page 2)
Tightening torque:

21 N·m
2.1 kgf-m
15.2 ft-lb

I used Denso part: 234-9123 (it comes with a pack of anti-seize in the box)

As of Sept. 8, 2019, it was $89 at Rock Auto.

I used a split socket wrench and still managed to do the job. I think the crowfoot wrench would be a better choice if you don't have either and will buy a wrench. You'll need about 18 inches in extensions to get your wrench above the radiator so you can turn it.

I had a P2097 code. The car had smelled like raw fuel long before the check engine light came on, maybe for 5,000 miles. The car even passed smog while it smelled rich-burning. I could feel the car run smoother after I changed the sensor. It was an easy 15 minute swap out. Mine was not frozen on. I bought a breaker bar but didn't really need it. Total was about $120 for parts and tools. I hope this helps someone else here.
 
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