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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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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?

KATUR 22mm Ratcheting Wrench Set Flexible Pivoting Head Ratchet Combination Spanner Wrench Chrome Vanadium Steel Metric Tool (22mm) : Amazon.ca: Tools & Home Improvement

KATUR 22mm Ratcheting Wrench Set Flexible Pivoting Head Ratchet Combination Spanner Wrench Chrome Vanadium Steel Metric Tool (22mm) : Amazon.ca: Tools & Home Improvement

www.amazon.ca

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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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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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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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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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.

 

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Here is a view from above looking down at the air/fuel sensor (again green wire is cut off)

 

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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.

 

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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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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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Thanks for the suggestions Cory, but if you read through the thread you will see I got them out, and how. Not too sure what you are referring to with the rental tool high torque oxygen sensor socket. I own one. They don't always work as advertised.

 

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I can see how that might just be perfect for the job, and allow you access from underneath.