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2002 Subaru 2.5RS
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7 Posts
Discussion Starter #1
I am fairly new to everything Subaru and maybe I'm blind but I can't seem to find conclusive answers to a couple of questions. My car(2002 2.5rs with a 3in wrx catback currently) has a bad catalytic converter, and I am planning on going catless and doing either UEL headers or EL headers soon. (I haven't decided which yet) My question is do I need to run a special O2 sensor up front to compensate for this mod? I have heard of a wideband O2 sensor but I don't know if that term is referring to the front or rear sensor. The rear O2 sensor is brand new since the car had a bad sensor there when I bought it. Also, reading around my car is having some symptoms of a bad front O2 sensor, such as bucking when its warmed up, a popping sound on deceleration like a backfire but definetly not a backfire, and just overall weird behavior right when the throttle is applied after being closed while driving. The front O2 sensor is almost as expensive as the exhaust that I want to get so I want to be sure I get the right one. Knowing all of this, is there anything in particular I should look for in a front O2 sensor?

(if you're thinking of responding to my post with something like"just search, it has been documented" then prove it to me with a link, because I can't find a SOLID answer) :loser:
 

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Baby❤Daddy
98L Wagon
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27,848 Posts
get a denso OE replacement o2 sensor. do not get bosch. amazon is a good place to start.

EL versus UEL will depend on what you want it to sound like. EL will make it sound like a honda. UEL will make it sound like a subaru.

no, you do not need a special o2 sensor up front.

when you buy a header/cat section make sure you buy (or make sure it's included) a CEL fix for the rear sensor. it's an L shaped section that takes the sensor out of the direct stream.
 

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Baby❤Daddy
98L Wagon
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27,848 Posts
yup. and you can't go wrong with the OBX.

tsudo makes a decent kit that well priced as well.
 

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02 rs2.5, midnght blk pearl
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60 Posts
I have the same year of rs.. And a wrx cat back.. I pop all the time . What brand exhaust is it ??

But this "bucking" I don't understand..
 

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2002 Subaru 2.5RS
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7 Posts
Discussion Starter #6
The bucking is likely due to my front O2 sensor being bad and sending my ECU bad readings and causing it to adjust timing and fuel incorrectly resulting in poor performance. Thats my best guess according to my research on it anyways.
 

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Banned
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4,438 Posts
a separate wideband sensor/gauge (many options, AEM, Haltech, Innovate, Stack... accuracy vary widely among them as do sensor replacement costs) can be added into the mix for AFR monitoring, but cannot substitute your OEM sensor without using a standalone ecu (Haltech, etc)

for an OEM replacement sensor, replace the faulty cat first, reset the ecu, and see what it does. The clogged cat may be the source of all your issues
 

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2000 2.5RS/C BRP
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2,321 Posts
What's this "L shaped" section you spek of, B? :confused:
There's literally no good images, but this isn't bad. Next time I have my car on the lift (a few months, the Lotus is on it and there's snow outside) I'll take an in situ.



It screws into the rear O2 bung, the rear O2 screws into it.
 

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OG GC
MY00 BRP RS...still adding to
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4,471 Posts
There's literally no good images, but this isn't bad. Next time I have my car on the lift (a few months, the Lotus is on it and there's snow outside) I'll take an in situ.



It screws into the rear O2 bung, the rear O2 screws into it.
Awesome! Thanks for the image.
Now to go look for it. :noes:
Didn't have any luck at Amazon...
 

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OG GC
MY00 BRP RS...still adding to
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4,471 Posts
Ahh....I was on EBay looking up obx o2 bracket, obx sensor bung, obx L bracket...etc...:rolleyes:

Found some similar now. Thanks, Man! :)
 

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1997 Impreza Brighton
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491 Posts
a separate wideband sensor/gauge...[trim]... cannot substitute your OEM sensor without using a standalone ecu
Zeitronix makes a wideband controller that has a "narrowband simulator" output. I use this with the origninal ECU.
Apparently it can be programmed to change the "switching point" of the signal (where it tell the computer stoich is), but I haven't played with that feature yet (need to find the right serial cable).

But for the OP, a wideband is more trouble than it'll be worth in your case.
 

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2000 2.5RS/C BRP
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Zeitronix makes a wideband controller that has a "narrowband simulator" output. I use this with the origninal ECU.
Apparently it can be programmed to change the "switching point" of the signal (where it tell the computer stoich is), but I haven't played with that feature yet (need to find the right serial cable).

But for the OP, a wideband is more trouble than it'll be worth in your case.
You have a '97, which uses a true narrowband. Post 01, they used true widebands. In the middle years there, it was a half-wideband that's almost impossible to simulate. Additionally, there's two signal lines the ECU transmits inverse signals on to check that the O2 hasn't been tampered with, so you'd need two identical simulators.

Blech.
 

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You have a '97, which uses a true narrowband. Post 01, they used true widebands. In the middle years there, it was a half-wideband that's almost impossible to simulate. Additionally, there's two signal lines the ECU transmits inverse signals on to check that the O2 hasn't been tampered with, so you'd need two identical simulators.

Blech.

sorta, but not really

The sensors in later cars are kinda widebands - bone stock, they do not have the same range as aftermarket widebands. think their floor is 11:1, and their celing is around 16:1 (I forget the exact range, IIRC it's a 0-4 volt scale). That's not a wideband in the modern sensor of the term. But certainly better than a 0-1v scale. But the ecu on late model WRX/STI (I forget up until what year, as I've never had a reason to check modern ones) doesn't calculate it's tables off the wideband signal. I know earlier WRX's you can rescale the factory 02 tables, don't know if that's doable on later year cars.

This is why when tuning, you need a variety of data points, recording the same runs, so you can spot the trends, and use the data correctly. No one single data point (02, EGT, MAP, AIT) gives you a truly accurate picture.

The long and short of it: for closed loop, they all work fine. For open loop, a stock sensor indexed off a stock table ain't where it's at (location notwithstanding, as just like in real estate, that has a HUGE affect on outcome, especially on forced induction cars)

Anyway you slice it, as stated above, this is all way beyond the scope of what the OP likely needs for his proposed setup
 

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Zeitronix makes a wideband controller that has a "narrowband simulator" output. I use this with the origninal ECU.
Apparently it can be programmed to change the "switching point" of the signal (where it tell the computer stoich is), but I haven't played with that feature yet (need to find the right serial cable).

But for the OP, a wideband is more trouble than it'll be worth in your case.
True, but an emulator/simulator isn't necessarily quite the same. Would be interesting to plot/log the factory sensor off the Zeitronix and see the real comparison

Remember, a sensor is a tool: but the table/calibration of that sensor is what yields the results
 

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1997 Impreza Brighton
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sorta, but not really
Anyway you slice it, as stated above, this is all way beyond the scope of what the OP likely needs for his proposed setup
Still good to know, thanks for the clarification! :)
 

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2000 2.5RS/C BRP
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sorta, but not really
Okay, that's beyond what I played with with the stock ECU. I put in an '02 WRX ECU and haven't looked back yet.

But you didn't touch on the dual & inverted signals- to my understanding from even just the FSM that's the big issue. Do you know on that one?
 

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2002 Subaru 2.5RS
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Discussion Starter #20 (Edited)
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