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Old 09-07-2009, 12:19 AM   #1
magicstop
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Join Date: Mar 2009
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2004 WRX Wagon
TXS Stage 4

Default How To: Installing / changing your front o2 sensor

After doing some research and exploring, I've decided that a better, more comprehensive NASIOC guide for changing and installing a front o2 sensor is needed. The best guide I have found so far, written by WRXreX, is located over at ClubWRX, but I found it to be somewhat lacking for the novice. It can be found here:
http://www.clubwrx.net/forums/mainte...or-02-wrx.html
I will be using my own pictures and writeup, but will also be borrowing from WRXreX's writeup, as my vehicle isn't stock, and the procedure was somewhat different. Hopefully between the two, any noob who's never even done an oil change will be able to (with the right tools) change their own front o2 sensor.
Let us begin!

Tools you will need:
Car jack
Jack stand
Lug wrench
Ratchet
10mm socket
12mm socket*
3/8 inch ratchet extension
22mm o2 socket (or kit, as found from Autozone: Part #27110
2 zip ties*
New o2 sensor (Denso and Bosch make fine replacement sensors for under $200. Or you can get an OEM for $200+, but it's just a Denso . . . Your choice!)

* - indicates tools that were used on WREreX's install, but not on mine

Your front o2 sensor is accessed from behind the front passenger wheel. You'll want to remove the wheel to have easy access to it.



The first thing that I do when working with any electrical component of my vehicle (although in this job, I'm sure it's unnecessary), is to disconnect the negative terminal of your car's battery. Pop your hood, and use a 10mm socket to loosen the clamp. Pull it off, and move on! Don't forget to come back and hook this guy back up.

Next thing to do, before jacking your car up, is to loosen up the lug nuts on your front passenger wheel. Doing it while the wheel is on the ground prevents the wheel from turning while you try to untighten them. This is the hardest part of the job if you don't have air tools or some other means of cheating. Lug nuts, rightly so, are on there TIGHT. Again, probably useless, but I prefer to tighten and untighten lug nuts in a cross pattern, loosening first one lug nut, then the one opposite of it, and so on.

Once you've loosened the lugs nuts, using your preference of jacks, get your car up in the air (I use both the lever-style jack here as well as the tire jack that comes w/ your vehicle). Get a jack stand under there once the wheel is up to insure it's not going to come crashing down on you. You won't be under the car for the job, but you still don't want any trouble.



Now go ahead and finish taking off the lug nuts. Once they are off, pull your wheel, and you will have the following in front of you:



Now you can access your front o2 sensor easily. Get down and look behind your rotor to the right. You will see the following:



As you can see, circled in red, there are three pins in the mudguard that you will need to remove to get to the o2 sensor. The 4th on the bottom (not circled) can stay put. You can just swivel the entire flap on it.
For the top pin, I used a flathead screwdriver to pop it off. For the middle two pins, they can unscrew with a phillipshead.
Popping the three pins and moving aside the flap will reveal your o2 sensor!



Now you're ready to remove it. The o2 sensor is removed by unscrewing it using a 22mm socket. Autozone sells kits for $25 the include the socket, and two offset sockets (which is what you'll need). You can probably just find the offset socket alone somewhere, but I couldn't. The beauty of this, however, is that you can get it through the Autozone loan-a-tool program. You'll have to pay the $25 up front, but then you can return the tool when you are done, and you'll get your money back. Pretty sweet for tools like this that you'll probably not use again (or often).
Call your Autozone and ask if they have part #27110. Here's the item page for it: o2 Sensor Removal Kit
It's hard to get at this thing with a ratchet and socket. Your best bet would be to use an extension on your ratchet to get back to it easily. That's how WRXreX did his:

I didn't have an extension, and my spacing was a little tighter, but fortunately, I was using a ratchet with a swiveling head which made this possible. You want to play with it and figure out your best option, but ultimate, you just need to be able to unscrew the little bugger (and put a new one back in!).





Unscrewed o2 sensor:

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Old 09-07-2009, 12:19 AM   #2
magicstop
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Join Date: Mar 2009
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2004 WRX Wagon
TXS Stage 4

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The next step is to unplug the o2 sensor. Your hood should still be open from disconnecting the battery. I suggesting getting a hold on the o2 sensor and jiggling it, watching where it connects under the hood. It should be on the passenger side, by your intake. On my setup, with the TXS Shorty intake, it was very visible and very easy to find the harness. It's by the boost solenoid.





A picture of it from WRXreX's guide:



Simply depress the little plastic level and pop out of the old o2 sensor's wiring. On my system, at this point, I was good to go. All I had to do what pull the o2 sensor out from behind the wheel, wiring and all.



However, if your setup is stock or you haven't changed the wiring in this area, you've got one more step to free your wires up. Evidently there is a bolt securing a bracket which is connected to the o2 wiring via two clips. You'll need to remove your intake box to get at it, remove the bolt and bracket, and use zip-ties or reusable clamps to secure your wires.

Picture from WRXreX's guide:



Once you've done that, pull your old o2 sensor from behind the wheel, wires and all.

Now that it's out, reinstallation is a simple reversal. I started from under the hood and dropped my o2 sensor down from where it's harness connects in. That way I didn't have to tediously feed it up through a bunch of crap to get under the hood. Clip the sensor in, secure it how you best see fit, and make sure it's not resting on anything that will get really hot! You don't want to melt your wires!



Then find and pull the o2 sensor through and screw it back in with your 22mm tool.



Once it is secure, double check your wiring, and if all is well, go ahead and cover the mudguard back up, securing the pins as they were before. Just a note: I had trouble with the top pin. The phillip's head pins are easy, but I couldn't get the top one in. I just left it, as it felt secure enough.



Once covered, put your wheel back on tighten it, lower your car back down, and the securely tighten the wheel even further until you know it won't fly off on the interstate!
Now simply reconnect your battery, and you're set! Job is complete, you are richer for having done your own work, and it's pretty damn satisfying!

I'd like to thank WRXreX over at ClubWRX for making the guide that I used to do my own install (http://www.clubwrx.net/forums/mainte...or-02-wrx.html). I hope NASIOC users and other folk will find this guide useful and thorough. If you've got any suggestions, complaints, or qualms about it, let me know and I'll do my best to adjust.
Thanks!
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Old 09-07-2009, 12:26 AM   #3
Un0RiGiNaL
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Default

nice diy


for me, its alot easier, i lift the car, and i just remove the o2 sensor, its visible and easy. then i just drop the other sensor thru the engine bay.
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Old 01-21-2011, 09:31 PM   #4
vogankhadafi
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Awesome. Thanks!
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Old 01-21-2011, 09:51 PM   #5
Badler
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I'll be using this. Thank you!
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Old 01-27-2011, 10:24 PM   #6
txl146
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Great guide - I am WRXreX by the way.
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Old 01-28-2011, 01:05 AM   #7
leecea
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Are you sure that suggestion to use a long extension will work? It doesn't look right to me.
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Old 02-08-2011, 09:32 PM   #8
exchelente
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Black

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Thanks, nice write up! Props to you and WRXreX (txl146)

Changed mine a few days ago using these instructions and it went well. Most annoying part for me was not having a long enough extension to get the airbox bolt out.

Got a nice deal on the part from Amazon for my 02 wrx - http://www.amazon.com/gp/product/B000C5YD1A/ref=oss_product$117.68 & this item ships for FREE with Super Saver Shipping

dealer wanted $300 for the part alone (plus 1 1/2 hr labor,) pep boys wanted $188 (and it wasn't a denso, the oem)

I picked up the offset socket for $10 at autozone. I probably could have gotten it cheaper at harbor freight but it's a further trip.
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Old 02-09-2011, 12:20 AM   #9
mobomelter
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Nice write up. Though I did mine without removing the tire because I am lazy.
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Old 02-09-2011, 01:42 AM   #10
Tubbs540
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Hey man awesome DIY...

I think we need more of em on here.
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Old 06-07-2011, 12:50 PM   #11
05DoubleURX
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Great how to..used this thread after I threw a CEL P0031. I replaced the front O2 sensor and it fixed the problem and saved myself some $. Thanks!!
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Old 03-13-2012, 08:29 AM   #12
02wagonrex
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wrb

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How do you know if you need this? I'm not throwing any CELs but I'm only getting 21mpg at best!
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Old 03-13-2012, 08:46 AM   #13
Wagon Of Fury
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Quote:
Originally Posted by 02wagonrex View Post
How do you know if you need this? I'm not throwing any CELs but I'm only getting 21mpg at best!
When you are throwing the indicated code.

I changed mine around 60k to clear the code and pass inspection, my mileage didn't change a bit.

Chances are other factors (including a lead foot) are to blame. Or you have a badly tuned/modded car !

Also, VatoZone will rent you the tool for free, and RockAuto has the part for about $115 and if you search the interwebs you can find a 5% off coupon which should cover shipping.
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Old 03-13-2012, 09:04 AM   #14
DavC825
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Slowest 07 STi

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That socket that you used on it is a life saver!!!!

I don't like how subaru made the heat shield, you can't pull it off without putting off the sensor first. I ended up notching it so it can be removed without pulling off the sensor.
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Old 03-13-2012, 09:35 AM   #15
02wagonrex
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I'm positive its not my foot. Car is stock. Wtf can it be?! I have done soo many searches!
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Old 03-13-2012, 09:39 AM   #16
SillaceSTi
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Quote:
Originally Posted by 02wagonrex
I'm positive its not my foot. Car is stock. Wtf can it be?! I have done soo many searches!
Are you doing a lot of city driving? Driving around in the city drops my MPG from approx. 25 down to around the 19/20 neighborhood.
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Old 03-13-2012, 10:15 AM   #17
02wagonrex
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70/30 highway city. Even 100 percent highway never got better than 21.6. Recent average has been 19
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Old 03-13-2012, 10:42 AM   #18
02wagonrex
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about 20k ago i replaced plugs and air filter, cleaned MAF,use synthetic oil,use seafoam and mystery oil on a regular basis. changed rear diff with royal purple about 1k ago and i went from 290-320 a tank(so my wife says lol) im about to do another tune up but i have never gotten better than this. why? my last plugs looked normal. ill check these when i take them out. no CELs and im lost. why so low? its killing my wallet! i read that this O2 can cause this?
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Old 03-13-2012, 03:24 PM   #19
Wagon Of Fury
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Quote:
Originally Posted by 02wagonrex View Post
about 20k ago i replaced plugs and air filter, cleaned MAF,use synthetic oil,use seafoam and mystery oil on a regular basis. changed rear diff with royal purple about 1k ago and i went from 290-320 a tank(so my wife says lol) im about to do another tune up but i have never gotten better than this. why? my last plugs looked normal. ill check these when i take them out. no CELs and im lost. why so low? its killing my wallet! i read that this O2 can cause this?
Well it's a cheap fix and you sound like you can handle it yourself, so there's no harm in trying, but just don't expect the world of difference.

Good luck !

My 03 WRX Wagon never got better than 22-24 MPG, city or highway, driving like a granny or a maniac, stock, Stage II, Stage IV, then back to stock. Such is life.
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Old 06-27-2012, 09:43 PM   #20
begbudii
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Do these instructions work for the 2003 model as well?
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Old 06-27-2012, 10:35 PM   #21
575rider
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Quote:
Originally Posted by begbudii View Post
Do these instructions work for the 2003 model as well?
yes..
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Old 07-16-2012, 12:57 AM   #22
LastResort
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Replaced mine this weekend at 100k, with a P0031. Bought the Denso sensor from Amazon, along with the offset socket and things went pretty smoothly despite all the 'help' from my 2 year old. I figured that the getting the sensor out would have been the hardest part, but it took way longer to get to that bracket than the two zip ties deserve. Go figure.
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Old 07-16-2012, 07:59 PM   #23
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Quote:
Originally Posted by 575rider View Post
yes..
Great instructions, just did this on my '03 with 165,000K. Was getting a code, low catalytic threshold. Was told a 'lazy sensor'. Dealer wanted to charge me $700 for front and rear. Got 'em on Amazon for $185 for both, freakin' dealers.
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Old 08-24-2012, 01:28 AM   #24
SubieMonte
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Going for it this weekend!
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Old 08-25-2012, 09:42 AM   #25
SubieMonte
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Got it out! I'd say that was easy. I did have some major engine work done about 15,000 miles ago. I would think that the O2 sensor would have been removed. Guess I got lucky on that. Anyway, here's the good and bad. iPod doesnt get crisp up close photos. Check out that blue wire hanging on by his chinny chin chin!



Time to put it all back together!

Edit....and done! Running like day 1 at 98,500!!!

Last edited by SubieMonte; 08-25-2012 at 12:44 PM.
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