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2011 Subaru Forester
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Discussion Starter #1
Firstly I am surprised that almost all the info here is for first and second-gen Foresters. That said in the quest to engage in some preventative maintenance, I tried to remove my upstream O2 sensor ( technically air-fuel sensor), and the mother will not budge, and my O2 socket is rounding it off.
I did as the service manual instructs and disconnected my right side engine fan so that you can angle a couple of long extensions down and get at the O2. But despite lots of liquid wrench, it was no go. I did it with the engine and exhaust cold, but I might have to try with it hot.
My question is should I just try to bust the sensor so that I can get a short 6 point socket on it? I don't like using heat and am not too sure how you would in any case unless you drop the whole exhaust. How hard is it in fact to drop? That might have to be the other alternative.
And it's no longer preventative maintenance after my failed efforts, idle is rough, code P1153 and despite clearing that and doing an idle relearn idle is still too low and wants to possibly stall. The car has 260,000 km, the sensors were original, and while they were not giving off error codes I figured they were on the downslope and it would be a question of time
 

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2011 Subaru Forester
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Discussion Starter #3
I think my post actually answers that one. I know where the sensor is. In addition to attacking it from above, there is a possibility of getting at it from underneath with a 22m wrench, the electrical connector actually fits through it. However, you have maybe an inch of play, and not the greatest angle especially when you have already rounded the sensor nut a bit.
I think I will probably end up having to break it to get a short 22mm 6 point socket on it, its what had to do on my previous X trail to replace the upstream though the set up was different. It just makes me a bit nervous to deliberately bust it.
How hard is it to drop the exhaust from the front with the two front wheels on-ramps? And if I do should I have replacement exhaust manifold gaskets on hand. The sensor would be easier to remove with some heat applied, but there is no way to do it with the exhaust in place.
For anyone doing this the torque value for the sensors is 15.5 ft-lb, and the service manual says to remove the right side engine fan. First, you disconnect the battery, and then undo the plastic undercover--one 12 mm bolt and two clips one of which is in the passenger side front wheel well. Then you disconnect the fan and the upstream and downstream O2 sensors' electrical connectors ( you press on their tabs to release them) which are on the right side. You undo and remove the air intake shroud, and undo the two 10mm retaining bolts for the fan on the right side. With the fan released you cannot fully remove it without removing the top coolant hose, but you can move it a bit lower and to the right to give you a better view and access to the upstream sensor. With two 1 foot extensions, you can get an O2 sensor socket on it. Unfortunately, the long one I have is a 22mm or 7/8 so in truth, it's a 22.25mm, and the grip is not quite right.
After I do get it done, fingers crossed, hopefully, the downstream is easier, I believe it's necessary to perform an idle relearn.
 

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06 FXT 4EAT, 09 X
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When I did mine I did it from the top with just removing the air shroud. I would assume an 09 and 11 are pretty much the same.

Cut the wire close to the sensor so it's out of the way, stick a wrench on the sensor and smack it with a large hammer.
 

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2011 Subaru Forester
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Discussion Starter #6
Yeah, wrench from underneath is the way to go. That's how I did it. Also, don't be afraid to hit it with a torch it's necessary sometimes.
I tried that after rounding the sensor a bit, but my 22mm is a 12pt and it just did not work-- too much rounding. Maybe I could finesse a torch if I had a proper lift but no way I am contorting and trying to get an open flame in there.
For the wrench maybe with one of these?

It should be a lot easier using the wrench on the O2 sensor but that upstream air/fuel sensor not as much. Still, I am encouraged by the fact you got yours out.
 

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2011 Forester X
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114 Posts
Your basic 2 prong open ended wrench is best. And hit it (the wrench) with a rubber mallet. Try to break it free. If the sensor is rounded beyond use you can get a metal file and try to straighten out the edges. Then find a wrench that fits or tap on a smaller size wrench. If you don't wanna try the torch you can heat up the exhaust by running the car. Should help the liquid wrench penetrate. It will smoke. Just watch your arms.
 

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2011 Subaru Forester
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Discussion Starter #8
When I did mine I did it from the top with just removing the air shroud. I would assume an 09 and 11 are pretty much the same.

Cut the wire close to the sensor so it's out of the way, stick a wrench on the sensor and smack it with a large hammer.
Pretty similar, but different engines. Thanks for the encouragement. I am giving a rest for today, but will try again tomorrow.
 

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06 FXT 4EAT, 09 X
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Your basic 2 prong open ended wrench is best. And hit it (the wrench) with a rubber mallet.
Closed end is best if you're going to hit it with a hammer, assuming it's not too rounded for that and you're willing to cut the wire or can fish it through. For mine I resorted to using a 2.5lb sledge hammer, so rubber may not cut it.
Don't be afraid to break a wrench, wear safety glasses.
 

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2011 Forester X
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Whew no I did not need a 2 pounder lol. Yours must have liked where it was. I torched it a bit while it was hot from running and was able to get the wrench on and smack it with the hammer to get it free. If its rounded I would go with the open end.
 

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Discussion Starter #11
I just can't figure out how you guys got a wrench in there, let alone the ability to file the stripped edges. My 22mm wrench is straight and about 12'' long and there is just no angle of attack. Super easy freeing the downstream O2 sensor, but my upstream is even more rounded. Tried with the wires cut to get a 7/8 full length 6 pt socket on and its turning as well, tried to force a 21mm socket on and no go.
I have tried to break the sensor with no real luck. It sure stands up to abuse.
I will try a bit longer but I think I will need to have it towed to a garage who will have to drop the exhaust to get the damned thing out. Anyone know off hand if you can drive the car with the upstream air fuel sensor and downstream 02 sensor disconnected?
 

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06 FXT 4EAT, 09 X
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I used a much shorter wrench. You might try vice grips but that's probably going to make it worse if you can't get them tight enough.
 

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2011 Subaru Forester
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Discussion Starter #13 (Edited)
Vice Grips don't work either, but it's done. Breaking off the end allowed me to get a full six-point long 7/8 socket on it. (Without breaking the metal tip it was too long for the socket and not letting it seat properly). I added a bit of electrical tape to the socket, got a good angle, two-foot pipe extension on the breaker bar, a prayer, and eureka.
Three and a half hours today on this. Now going to have some food, and then start it up and do an idle relearn.
I Will post some pics later on. By the looks of the sensors, they had seen better days.
 

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2011 Subaru Forester
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Discussion Starter #14 (Edited)
Here is the view from underneath. O2 sensor is to the left and air/fuel sensor to the right (wires cut off).
Note I did the job with the vehicle on the ground, and wheels turned to the right.
543164
 

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2011 Subaru Forester
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Discussion Starter #15
Here is a view from above looking down at the air/fuel sensor (again green wire is cut off)
543165
 

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2011 Subaru Forester
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Discussion Starter #16 (Edited)
And finally here are the removed sensors, with new ones under them still in bags. I have been running rich. Pretty sure these were on their last legs at 260,000 kms, even if no codes had been set. I am assuming they had never been replaced.
Car is running nicely, no codes, idle still a bit off but it seems to be learning. I will do a proper idle relearn tomorrow or see how goes for a couple of days. Just glad its done.
543166
 

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2019 Forester Limited
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Frustrating when a simple appearing job goes on and on, but so satisfying when, finally, success. Way to stick with it. 👍. Any bloody knuckles?
 

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2011 Subaru Forester
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Discussion Starter #18
Happy to say not even a cut. I am getting smarter and wear gloves. If I ever do this again, I won't even hesitate to bust the sensor. I wonder if one of the crowfoot style O2 sockets would have worked better than the long cut out socket version? I have an appreciation for why pro-mechanics have extensive toolboxes.
Thanks for the encouragement, and a shout out to Syphers47 and Aguy for their suggestions. I did let it run to get hot, and then sprayed Liquid Wrench, which probably helped it penetrate and do its job finally, but it had cooled down by the time I removed the sensors.
 

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2011 Subaru Forester
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Discussion Starter #19
Just to post a follow up. It has taken 3 days for the idle to fully reset, but it's running better than it ever has since I have owned it. The engine is really responsive, idle steady, and correct at stops. Smooth as well, and my bit of engine vibration at low idle is gone.
If you have a higher mileage FB25, and they have never been changed do yourself a favor, and don't wait to replace them until one fully fails.
 

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2005 Forester EJ251
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I have a 2005 Forester and the sensor is in a different position than your car. I purchased a number of special oxygen sensor sockets and none would fit as there was not enough space ti slide them on. I ended up using a 22mm flare nut spanner and the sensor came out easily. If necessary, give it a tap with a rubber mallet (rubber hammer). Also, no way would a tension wrench fit to tighten it. You only need the slightest of force on the spanner to achieve the 15 ft/lb torque. You do not want to tighen it too much as you might have to take it out some time later down the track.
 
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