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Old 05-24-2004, 09:18 PM   #1
AcquaCow
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Question Understanding o2 sensors...first vs second, high flow cat & CEL's?.

I've noticed that there are 2 sensors connected to my exhaust setup.

One up at the top of the cat section, and one furthur down.

Do these compare the before/after effects of the cat's and from there decide if the cats are working or not?

I know that there have been several "fixes" for the o2 CEL's (Resistor, extra bung/etc..)

I've been wanting to swap the exhaust system, but the idea of driving with a CEL on all the time has held me back.

If these sensors are indeed for before/after comparison, is it possible to get an efficient high-flow single cat to replace the stock setup that wouldn't bother the ECU?

I know that people haven't had much luck running catless, but I don't want to run catless. Would a good high flow cat throw a CEL?


Anyone with a CEL-less high-flow setup want to comment?

If you had a CEL with a high-flow, were you able to get rid of it?

Thanks,

-- Dave
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Old 05-25-2004, 11:52 AM   #2
Tim Sanderson
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I can't recommend a cat but I can tell you that your other assumptions are correct.
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Old 05-25-2004, 12:04 PM   #3
DreddRS25
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Old 05-25-2004, 12:58 PM   #4
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The front O2 sensor ensures that the computer is mixing the correct amount of fuel (using the injectors) with the measured amount of air flowing through your T/B. This is why it is also called the air/fuel sensor.

The rear O2 measures cat light-up time (if it fails, lights the P0420 we all know and love) and checks the work of the ECU and front O2. Ideally, the rear O2 readings vary very little from stoich, while the front O2 sensor may vary more from stoich. The cat is in between to clean up any richness before the rear O2 samples it.

When replacing exhaust components, you're fouling Subaru's work. When including a cat, though your exhaust is probably still inside sniffer limits, you've introduced many new variables the system wasn't designed to cope with.

Quicker cooling of exhaust gases (no replacement exhaust system is made of double-wall stainless with heat shields) is but one cause of CE lights.

Depending on your year of manufacture, and as you have an '00, whether it is early or late, you may be able to replace exhaust components without triggering an CE light. Subaru changed ECU logic, especially as relates to the rear O2, somewhere in the '00 MY. I have a late car, and an exhaust, and a CE light. It's not that big of a deal. If you are worried about it, you could also purchase an OBD-II scan tool to check the codes one a week. That's what I told myself I'd do, but after about a month I quit that and just check now and then.
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Old 05-25-2004, 02:21 PM   #5
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So is the rear sensor preset to look for a certain level of emissions and only turns the P0420 on, or is it sending back more information that the ECU processes? I've always wondered if those CEL fix kits were shutting off more than post-cat emissions (in a pass/fail format) to the ECU. I've had a front sensor go once so I know how critical they are to the car, but the rear is still a bit of a mystery to me.
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Old 05-25-2004, 05:11 PM   #6
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Rear O2 is an OBD-II addition to measure the health of the cats and front O2 sensor. They don't control mixture functions.
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Old 05-25-2004, 09:50 PM   #7
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What you want is Flat4's O2 extension. Go to:
http://www.flat4engineering.com/main/index.htm

and click on Products --> turbo --> up/down pipe and then look at the second to the last item.

I run this with a Borla header & CAT back and a Random Tech CAT. No CELs. I had the headers wrapped at one point to assist in scavaging, but torn off the wrap AutoXing. No problems so far.

HTH,
Chris
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Old 05-26-2004, 11:12 AM   #8
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Quote:
Originally posted by Julian
Rear O2 is an OBD-II addition to measure the health of the cats and front O2 sensor. They don't control mixture functions.
I know it doesn't affect mixture (I drove with an unplugged Honda sensor in there when I was in a bind once), I'm interested in how the rear sensor monitors emissions. What is it sending to the ECU; varying data the box uses to determine turning on the CEL? Or is it preset to send an "on" signal when a level of emissions sensed? I would guess the former if the ecu can detect sensor failure, but I don't know for sure.
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Old 05-26-2004, 01:42 PM   #9
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Quote:
Originally posted by AngryBlueRS
What you want is Flat4's O2 extension. Go to:
http://www.flat4engineering.com/main/index.htm

and click on Products --> turbo --> up/down pipe and then look at the second to the last item.

I run this with a Borla header & CAT back and a Random Tech CAT. No CELs. I had the headers wrapped at one point to assist in scavaging, but torn off the wrap AutoXing. No problems so far.

HTH,
Chris
I didn't need it. I just got a brand new rear O2 Sensor and I get no more CEL's.
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Old 05-26-2004, 02:13 PM   #10
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Quote:
Originally posted by Yotsuya
I know it doesn't affect mixture (I drove with an unplugged Honda sensor in there when I was in a bind once), I'm interested in how the rear sensor monitors emissions. What is it sending to the ECU; varying data the box uses to determine turning on the CEL? Or is it preset to send an "on" signal when a level of emissions sensed? I would guess the former if the ecu can detect sensor failure, but I don't know for sure.
I think the sensor itself is just a voltage regulator...depending on the amount of whatever it's designed to fluctuate with.

IE: if there's x amount of oxygen in the stream, y voltage will be read.

The ECU would just put the data from that sensor in a table and compare it to allowable tolerances. Anything outside that range and it would throw a CEL.
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Old 05-27-2004, 04:16 AM   #11
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You can replace the cat-back section of the exhaust without causing any CEL problems.

Random Technology makes a high flow cat that is designed to fit in place of the 2 stock cats. I imagine if you search for their name you'll find more info. There are plenty of people here running the Random Tech cat, and I don't think I've seen complaints about the cat causing a CEL. Then again, I haven't paid much attention.

Pat Olsen
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Old 05-27-2004, 05:41 PM   #12
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I got a Random Tech Cat about a yer and a half ago (the one with the little buldge for the cat). Lots more noise, power, and a CEL for the rear sensor right away. I'm using the regular cat again just because it's quieter and uses less gas.
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Old 05-28-2004, 08:26 AM   #13
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Well, then, perhaps I'm wrong. Although I'm confused by your comment that it was "the one with the little bulge for the cat". The pics I've seen of the Random Tech setup for the N/A Imprezas has a "normal" cat in it, as seen here: http://www.randomtechnology.com/new_products_imp.html . From looking at their website it appears the WRX cat replacements use the smaller, bulge-in-the-pipe style metallic substrate cats, as seen here: http://www.randomtechnology.com/subaru.html .

Speaking of which, I ordered one of the metallic substrate cats this week to replace the cat on my MRT exhaust. I went with one from here: http://www.rpmoutlet.com/powercat.htm - the 2.5" flavor. Hopefully I'll be able to get it welded in this weekend, I'm curious how it will affect sound, power, and the ol' CEL. (I get CELs all the time from my Cobb cams, so that's not a big deal...)

Pat
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Old 05-28-2004, 08:50 AM   #14
Keith Cournoyer
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Default high flow cat

Hi Pat, I have the MRT's too, and will be needing a new cat in the next few months, can you keep me posted on your results with that cat your getting. Looks like a good purchase, Thanks,
Keith
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Old 05-28-2004, 11:08 AM   #15
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So, if my car only had the one O2 sensor in the front of the cat (which it does), then anything I did after the sensor would not matter to it? I could go catless, gutted, hiflow or whatever and get no CELs?
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Old 05-28-2004, 11:16 AM   #16
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So, if my car only had the one O2 sensor in the front of the cat (which it does), then anything I did after the sensor would not matter to it? I could go catless, gutted, hiflow or whatever and get no CELs?



Yes.
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Old 05-28-2004, 11:23 AM   #17
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Old 05-28-2004, 12:45 PM   #18
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Quote:
Originally posted by Patrick Olsen
Well, then, perhaps I'm wrong. Although I'm confused by your comment that it was "the one with the little bulge for the cat". The pics I've seen of the Random Tech setup for the N/A Imprezas has a "normal" cat in it, as seen here: http://www.randomtechnology.com/new_products_imp.html . From looking at their website it appears the WRX cat replacements use the smaller, bulge-in-the-pipe style metallic substrate cats, as seen here: http://www.randomtechnology.com/subaru.html .
The picture of the RS Cat is either old or they've redone it again. Copyright on the page is 2001 so maybe it's just a little out of date. Five minutes looking for a picture of what I've got wasn't successful, so you'll just have to imagine a pipe in the same shape as the stock cat but with a buldge in it for the purifying stuff.
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Old 05-29-2004, 07:50 AM   #19
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Here's my experience with getting CELs so far... it's gonna be a bit long-winded.

I have a 2000 RS with Borla headers -> random tech high flow cat -> stromung stainless midpipe -> prodrive axleback.

When I first was doing my exhaust, I undid my rear O2 sensor and found that the threads were damaged. My only option at the time was a Walker wire-in replacement sensor. When I started up the car I got a CEL. After doing some research past Walker's useless documentation, it appeared that I had miswired the sensor. I rewired it, disconnected the battery for a bit and started the car back up. CEL came back again immediately.

I did some research about mechanical/electrical CEL fixes and decided to try the Flat4 mechanical style fix first. A member of the board (00RScoupe) had done some extensive chronicling of his CEL woes over on rs25.com so I tried following his advise. I tried drilling out a Help! spark plug anti-fouler to get the sensor out of the direct flow of the now more turbulent exhaust but that didn't seem to work either. From what I read, using a 1/2" bit doesn't seem to open up as much room for a decent gas pocket as the Flat4 one mentioned that he ends up using.

That brings me to a couple days ago. I had ordered a direct replacement prewired Denzo sensor from oxygensensors.com and was going to try swapping that in. I had also used my Dremel to remove a significant amount of material from the anti-fouler to see if that would help. Before I started working on the sensor, I replaced my broken hood release cable and had to remove the battery for a bit to get the new cable in. When I hooked it back up and turned the car over, no more CEL!?!?! So I am back to square one. I have a feeling that it will come back on shortly once the ECU is done adjusting itself. I have only put at most 100mi on the car since. If it does come back on though, I will experiment with my new O2 sensor and will likely throw a post up about it.

Stay tuned.
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Old 05-29-2004, 07:57 AM   #20
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Quote:
Originally posted by Yotsuya
I know it doesn't affect mixture (I drove with an unplugged Honda sensor in there when I was in a bind once), I'm interested in how the rear sensor monitors emissions. What is it sending to the ECU; varying data the box uses to determine turning on the CEL? Or is it preset to send an "on" signal when a level of emissions sensed? I would guess the former if the ecu can detect sensor failure, but I don't know for sure.
Look through my post above and check out the link to 00RSCoupe's thread on rs25.com. He did some testing with an OBD-II scanner plugged it. The sensor sends a varying voltage to the ECU depending on the level of oxygen it senses. It seems that if the sensor goes out of the acceptable range for a over a certain amount of time, the PO420 is triggered. I believe this is how the mechanical fix works. You basically trap a pocket of exhaust gasses around the sensor that has a more constant level of oxygen in it, thus a more constant voltage is read.
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