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DIY-Oxygen (O2) Sensor Installation - Rear Upstream A/F aka P2251
Compliments of triax37 @ http://acurazine.com
3-15-2011
Hey everyone - I just replaced my rear upstream O2 sensor (after 90k miles, P2251 popped on), and thought I would do a quick DIY since I didn't see an official one. The difficulty is probably a "2" (scale of 1-5), and would cost $100+ labor from the dealer. It took me about 1 hour. For what it's worth, my freeway mileage increased from 25mpg to 32mpg:
Part: O2 Sensor - 36531-RCA-A02 SENSOR, LAF ($120'ish)
Tools: 10mm, 12mm socket wrench, and O2 socket wrench or large adjustable wrench, arms and hands
Thanks to nfnsquared for help getting my thoughts together and for the service manual scan:
http://tl.acurazine.com/forums/showthread.php?t=808497
Remove strut tower bar
1. Three 12mm bolts on left side and three 12mm bolts on right side.
2. Drivers side - there are two 10mm bolts (blue arrows), and 2 plastic clips holding cables to the strut bar - I didn't take a picture of the clips, but they are pretty easy to see.
3. Passenger side - there is one 10mm bolt (blue arrow) holding the bar down. There is a clip obstructing access to the pictured bolt, so remove this. After you remove the clip and bolt, swivel the part attached to the firewall (red arrow) so that it is out of the way of the strut bar bracket.
4. Now, remove the strut bar. It may require some jiggling - just keep pulling.
Remove the old O2 sensor
5. Find sensor - it's located in Bank 1 of the engine, on the passenger side, between the engine and firewall. You'll see it - trust me. The sensor is pointed towards the ground, with wires pointed in the air.
6. The sensor unscrews with an O2 socket wrench, or (if you're lucky like me) a large adjustable wrench. A socket wrench or adjustable wrench will fit down there, so lean against the passenger fender (cover it with a workmat if you have one), and get your head and arms in there. Some (like me) had better luck with a warm engine, but it's just an option. Lefty-loosey...and once you get some give, don't unscrew it all the way.
7. Now unclip the harness - there are two mounting points, (i) a piggytailed clip facing the engine, and (ii) the standard O2 sensor harness clip facing the cabin. Do the one facing the engine first - push the metal piece away from the harness, towards the engine, and slide off the O2 sensor harness off the piggytailed connection. Now that the harness is free, you can easily unclip the harness. Unscrew the rest of the way, and remove.
* Unfortunately, I spent the most time on this because I didn't realize there were two clips holding it to the car. Once I removed the piggytailed connection, the other one was easy.
Install New O2 Sensor
8. DO NOT TOUCH the new O2 Sensor's head. If you bought a new sensor from Honda, it will have loctite on the threads, but not a bad idea to put some more on there. Hold via handle, and gently insert into engine. Screw / torque down to 33 lb.ft.
9. Connect harness.
Put everything back together
10. Slide back on to piggytailed connection.
11. Replace Strut bar, including ancillary screws and clips.
12. Reset ECU - I unplugged battery, and had my navigation/radio codes handy to get them functioning again.
3-22-2011
Flick - the simple answer is cost. A new upstream O2 sensor will run you $120 or so. If you're noticing a loss of mileage, the O2 sensors are common causes. If you're still getting 30 mpg at 70 mph, then your sensor is probably working fine.
Keep in mind the computer is always checking the sensors, so if there was a big problem, you would get a Check Engine Light.
5-13-2011
Re: Bosch - after searching around the 3G forum, I didn't find anything conclusive on Bosch/Denso. So I strolled over to the 2G forum, where people definitely had issues with the Bosch sensors. I ended up buying the Honda part.
5-16-2011
Jim - this post is to fix the code 2251. The O2 sensor is located on the passenger side of the car, in the back of the engine closest to the firewall. If you are standing over the passenger front tire, look to the left of the power steering pump and shine a flashlight down into the back of the engine. You'll see the O2 sensor - it's pointing straight up to the sky. Let us know how we can help.
