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Old 10-06-2013, 07:07 PM   #1
Aremel
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Join Date: Oct 2013
Location: High Point, NC
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2004 Impreza 2.5 TS

Default My oxygen won't sense properly!

Hello everyone.

Thanks for taking a few moments to read about my problems. First, this is basically a verbatim repost from the newbie forums of the post titled "Frustrating '04 2.5 TS - Front or rear O2 sensor?": http://forums.nasioc.com/forums/show....php?t=2558609. I just got access to the tech section here, so if this is bad practice please let me know. I want to contribute whatever I can. I'll post some pics of the entire process as well, as soon as I figure it out

In the meantime, if you're curious, I did post a few pics of my little ugly duckling here: http://forums.nasioc.com/forums/show....php?t=2558651. Here goes...

Long time reader, first time writer. I'd like to start off with many thanks to the community, moderators, and site owners for so much great information. These forums are invaluable to the DIY type. Please excuse me if this isn't the correct place for a tech post, but it wouldn't let me put it in the tech section b/c I'm a newbie... edit: I think I finally made it. Actually, it was really easy. Thanks y'all

I think I have come to an impasse with this issue and I'd love some advice. I have a 2004 Impreza 2.5 TS with around 140,000 miles.

Short version: Is the Denso 234-4700 an appropriate replacement for the upstream O2 (A/F) sensor? Internet searches for that part number say it is downstream!

Long version: I purchased said part based on a lookup by the retailer in their database. After installing the part, I get a "P0131 Front oxygen (A/F) sensor circuit range/performance problem (low input)" error. Before the new part it was a P0030 code. The old sensor was open across the heater, so it was indeed bad.

At this point I have checked the new sensor for functionality (propane torch bench test) and it seems to be working. I have inspected all cables for damage. I have even removed the ECU and checked the runs for continuity to it. Everything checks out fine.

One possibility is that I did put a very small amount of dielectric grease on the connection, which I now know isn't the best thing for low voltage signal applications. I did so at the the advice of the same people who evidently sold me the wrong part! I know, I know. I have since cleaned the connection so I don't think that is the issue. It was a very small amount to begin with.

So, I know that my front sensor should be a wideband one, and I don't know if this part is correct. If it is correct, and the proper signals are making it through my connections and wiring harness to the ECU, does that mean the ECU is suspect?

Thanks in advance for any help! When I get this one solved, I'll give back to the community and post the codes and solutions in the appropriate area.

Again, thanks so much for the past and future help!

p.s. - Thanks to the greatest community of car enthusiasts in the world, I have returned the unit to the store for a full refund. I hate that it had to be listed as defective since it isn't defective, just the wrong part number. I tried to save money by making the connections myself and not getting the factory harness already installed i.e. I bought the "universal". Just so we're clear, please don't think that I can't make a good electrical connection and ensure it's waterproof(ness?). I can also test my connections an ensure that any connections I made aren't adversely affecting circuit resistance, or signal acuity. But, according to the pinout diagram for my ECU, I'm not getting the voltage values that the ECU wants to see.

So, since the retailer does not offer a "universal" upstream sensor, I will capitulate and buy the one with a cheap plastic harness already installed (for almost $100 more) unless someone can steer me in another direction. Many thanks to Cocoa Beach Bum for replying to my newbie forum on this topic. I'm following your lead and as far as I can see, that is $50 cheaper than my alternative.

You guys are the best. I promise I'll try to give back. I few blurry picture at the very least
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Old 10-06-2013, 08:43 PM   #2
EdHilario
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The correct part for the upstream O2 sensor from Denso is 234-9015.
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Old 10-07-2013, 08:55 AM   #3
Aremel
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2004 Impreza 2.5 TS

Default

Thanks for the info. I'll be ordering the correct part today.
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Old 10-07-2013, 09:27 AM   #4
Patrick Olsen
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1996 Impreza coupe

Default

As a general rule, if you ever have a question about a part number, go directly to the source. Most manufacturers have application look-ups on their website, like so: http://www.densoautoparts.com/find-m...icle-selection. 9015 for the upstream, 4700 or 4706 for the downstream.
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Old 10-07-2013, 12:11 PM   #5
Aremel
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Thanks! That's a good resource.
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Old 10-07-2013, 02:10 PM   #6
Back Road Runner
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Yeah, the O2 sensor terminology gets people mixed up. It would be nicer if everyone referred to the upstream component simply as an air/fuel ration sensor (AFR). The rear sensor is basically just a cat efficiency sensor that determines if the cat failed or not. Both are described as O2 sensors though, so you do have to take some care in the details. Unfortunately they screw into the same sized hole, although depending on the car, the location of the sensors, and wiring it may or may not reach the plug on the engine.

I'm glad you got it sorted out. I typically buy a lot of my car parts at RockAuto.com, and while the purchase is categorized under oxygen sensors, they do indicate which are upstream and which are downstream sensors. If you walk into most automotive stores, the clerk will usually ask which one also. You can't rely on this 100%, but most are knowledgeable enough to ask you specifically which one you're looking to replace. If you're just hopping online and buying your own, the need for knowledge falls back on you. At least now you know, and sorry that it's the more expensive one.
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Old 10-14-2013, 11:18 PM   #7
Aremel
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2004 Impreza 2.5 TS

Default

Well, it is all sorted out, now. I've put about 200 miles on it and no light. It even passed inspection this afternoon. It's nice to be legal.

It was worth the extra cost for the peace of mind. As an added bonus, I gained a much deeper knowledge of its operation and I really appreciate that.
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Old 10-14-2013, 11:20 PM   #8
EdHilario
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Good stuff.

I just replaced my front O2 (AF Ratio) sensor as well over the weekend.
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Old 04-19-2017, 01:41 PM   #9
MargaretM711
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Default '04 TS O2 Sensor

Hey there -

I've come to the conclusion that I also need to replace my upstream O2 sensor, and I'm having trouble finding resources for a TS - most of the videos and such that I'm coming across are for the WRX or STi. From what I can figure out so far, the O2 sensor is in a completely different location on the TS wagon, so what I'm finding isn't actually helpful. Mind giving me some insight into the best way to actually access it in order to do the replacement?
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Old 04-20-2017, 11:18 AM   #10
Patrick Olsen
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1997 Legacy 2.5GT
1996 Impreza coupe

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Quote:
Originally Posted by MargaretM711 View Post
I've come to the conclusion that I also need to replace my upstream O2 sensor, and I'm having trouble finding resources for a TS - most of the videos and such that I'm coming across are for the WRX or STi. From what I can figure out so far, the O2 sensor is in a completely different location on the TS wagon, so what I'm finding isn't actually helpful. Mind giving me some insight into the best way to actually access it in order to do the replacement?
Jack up front end of car, put on jack stands. Look at exhaust upstream of catalytic converter. Find O2 sensor. Use 22mm / 7/8" wrench to remove old O2 sensor and install new O2 sensor.

Seriously, locating the sensor won't be an issue once you actually get under your car and lay eyes on target. There have been a couple/few different O2 sensor configurations over the years (depending on which emissions version the car is), so I don't want to say exactly where it will be, but it will be very easy to find. The hard part will probably be breaking the O2 sensor loose; they can be stuck in there pretty good after years of heat cycling and corrosion.
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