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Old 08-30-2011, 12:12 AM   #1
williaty
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Default Do Not Use a Rear O2 Sensor Extender Bung/Anti-Fouler/etc With a 2005+ Subaru

OK, this one is coming back again because a couple of vendors offer what they call "O2 Sensor Spacers" that they incorrectly list as compatible with 2005+ Subarus. I'm getting PMs about these on a pretty regular basis. They look like this:



They're also available as spark plug anti-foulers or as little extension tubes welded on to the pipe.

Regardless of what they're called, what they do is to move the O2 sensor physically farther away from the main exhaust flow in hopes of tricking the ECU into not throwing the P0420 DTC when you run headers, a HFC, or a trackpipe. While on some cars they do actually prevent the P0420 DTC from being thrown, they cause fueling problems on all cars 2005 and newer. In 2005, Subaru switched to an ECU that uses the input from the rear O2 sensor to affect fueling in addition to simply monitoring the cat (as in 2004 and older cars). If you remove the rear O2 sensor entirely, mount it somewhere that isn't in the exhaust at all (don't laugh, I've seen it done), or even just move it out of the main exhaust flow, you'll develop strange fueling errors as the ECU tries to make the rear O2 see what it's programmed to achieve. The result of this is slightly worse fuel economy, a worse idle, and generally increased difficulty in getting the car tuned to a good state.

Summary: DO NOT USE O2 SENSOR SPACERS OR SPARK PLUG ANTI-FOULERS ON 2005 OR NEWER SUBARUS.


Also, it's worth noting that you can't use electronic "MIL Eliminators" like the ones from Pony Express on 2005 and newer Subarus either. Your only solution to getting a P0420 DTC on the newer cars is to disable it in the tune.
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Old 08-30-2011, 12:19 AM   #2
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First, Ty I would like to say thank for continuing to post technical threads on the forum. While my car is so unbelievably old that I can't actually use any of it, I still like learning about how this stuff works from someone that actually knows his ****.

Second, IBheyguyscheckoutmynewo2anti-foulerformy2006imprezaisn'titsweet?
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Old 08-30-2011, 08:48 AM   #3
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is the rear o2 used as a cat efficiency check?

i am mainly from the GM 3800 world and the rear o2 on the 01+ cars is used as a cat efficiency check. it does not just monitor the flow, but the car runs a routine which it changes the fuel mapping slightly to check the cat and to see if the o2 sensor is responding correctly.

or does it actually make fueling changes based on the rear as well as the front?
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Old 08-30-2011, 08:55 AM   #4
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+1 here.

I tried this for a day or two on my '05, and it does just about exactly what he said. It's obvious that it makes it run worse and doesn't solve the CEL. I pulled the extender off, and it's still in my garage somewhere. I'm back to stock now, but this is a "don't try this at home, kids. It doesn't work" message.
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Old 08-30-2011, 09:18 AM   #5
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Quote:
Originally Posted by Bio248
is the rear o2 used as a cat efficiency check?

i am mainly from the GM 3800 world and the rear o2 on the 01+ cars is used as a cat efficiency check. it does not just monitor the flow, but the car runs a routine which it changes the fuel mapping slightly to check the cat and to see if the o2 sensor is responding correctly.

or does it actually make fueling changes based on the rear as well as the front?
Post 1 answeres your question in detail.
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Old 08-30-2011, 12:13 PM   #6
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Quote:
Originally Posted by Bio248
is the rear o2 used as a cat efficiency check?

i am mainly from the GM 3800 world and the rear o2 on the 01+ cars is used as a cat efficiency check. it does not just monitor the flow, but the car runs a routine which it changes the fuel mapping slightly to check the cat and to see if the o2 sensor is responding correctly.

or does it actually make fueling changes based on the rear as well as the front?
It is used as a cat check like willity's post explained, but ALSO its data is used to monitor/adjust fueling. The specifics of what areas of the fueling tables out has an effect on are a mystery last I heard. The front still has a majority of the input on fueling but in 05 the rear sensor joined the game.
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Old 08-30-2011, 01:19 PM   #7
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So with the the rear O2 sensor in the game, 05 and up cars are SOL when it comes to headers?
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Old 08-30-2011, 02:00 PM   #8
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You guys suck at reading. The last sentence of the first post explains how to avoid the CEL on 05+ subarus
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Old 08-30-2011, 02:53 PM   #9
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Old 08-30-2011, 10:21 PM   #10
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Here's to a long and happy life at the NASIOC forums. :|
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Old 08-31-2011, 01:56 AM   #11
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op, do you know/have the logic for the functioning of the metering of the rear o2?

I would really like to see what it is doing with the rear values. Comparing it to the front? Comparing to the front and a preset range?
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Old 08-31-2011, 02:21 AM   #12
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The only guy I know of who has a good enough understanding of the Subaru ECU logic to answer that question got bought by Cobb. I think answers are unlikely.
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Old 08-31-2011, 06:40 AM   #13
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How does rear o2 bung placement come into play wth this? I'd like to get a high flow cat soon, but all there bung are by the mid pipe, not farther forward like the 05plus.
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Old 08-31-2011, 02:46 PM   #14
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You'll need an O2 sensor wire extender from Casper's. Works just fine. However, even with an HFC, you're almost certainly going to get a P0420 on your 05RS.
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Old 10-15-2011, 01:44 PM   #15
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To confirm Williaty's info, i've been running aftermarket EL headers & a high-flow cat for a few months, and the rear O2 sensor was mounted away from the exhaust flow.
Since the install, the car was generally running a little rich, and at steady highway cruise AFR dropped to around 13.9-14.1, definitely not the optimal 14.7 that we're looking for, and no amount of adjustments to the tune were able to correct this.
Along with this, 20+ minutes of steady highway cruising always brought on P2096 & P0137 codes, both related to the rear O2 sensor.

Yesterday, I went back to the shop that fabricated the headers and they fitted a new bung allowing the rear O2 sensor to be positioned in the exhaust flow. After resetting the ECU, and over 2 hours of logging so far, the AFRs are now pretty much perfect, and (so far) no CELs or pending CELs whatsoever This is without adjusting the tune to optimise things yet!

A little more evidence that the rear O2 is doing more than just monitoring cat efficiency....

Last edited by NH8991; 10-15-2011 at 04:01 PM.
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Old 10-18-2011, 09:29 PM   #16
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This should be a sticky for sure AND yes, having it removed by a tuner is the best way. I had a local tuner remove the DTC from my ECU a ways back and never had a problem thereafter. Took him all of about 10 minutes or less with a laptop
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Old 10-19-2011, 06:46 PM   #17
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im bustin' double 420 codes and i dont even have headers. extenders or anything! it pretty much only comes on while im on cruise. its good to know how it all factors into the fueling. that being said, i have to go reset my car now.
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Old 10-19-2011, 10:06 PM   #18
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Quote:
Originally Posted by msingell View Post
im bustin' double 420 codes and i dont even have headers. extenders or anything! it pretty much only comes on while im on cruise. its good to know how it all factors into the fueling. that being said, i have to go reset my car now.
That's because you've probably killed the cats blowing coolant and oil onto them due to the headgasket leak you've had for a year now and haven't fixed.
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Old 10-20-2011, 09:47 PM   #19
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^^ Well at least I don't have to worry about that issue since I have not cats, but hope to not experience a bad HG leak. I thought I had a small HG leak but nothing seems to have gotten worse, looked wet around the gasket at one point but maybe it was oil that spilled from the filler neck, who knows.
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Old 10-20-2011, 11:01 PM   #20
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Quote:
Originally Posted by kero View Post
^^ Well at least I don't have to worry about that issue since I have not cats, but hope to not experience a bad HG leak. I thought I had a small HG leak but nothing seems to have gotten worse, looked wet around the gasket at one point but maybe it was oil that spilled from the filler neck, who knows.
Your model year has a problem where it starts to leak very early from the oil return galleries on the bottom of the heads. Make no mistake: it IS a headgasket leak and you DO need to do somehting about it. You'll get no further warning before it converts to a much more serious oil-to-coolant leak.
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Old 10-21-2011, 09:19 AM   #21
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aw dammit. that oil leak on my car is the head gaskets? fuuuuuuuuuuuu

i thought it was the pan gasket and i didnt care then...
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Old 10-22-2011, 02:59 PM   #22
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Quote:
Originally Posted by williaty View Post
Your model year has a problem where it starts to leak very early from the oil return galleries on the bottom of the heads. Make no mistake: it IS a headgasket leak and you DO need to do somehting about it. You'll get no further warning before it converts to a much more serious oil-to-coolant leak.
Oh just great, have 46K on the car and way out of warranty, def do not have the money right now to get the HG replaced. Is there a recall or TSB about this that can help me?
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Old 10-22-2011, 03:04 PM   #23
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Talk to SoA. It's been hit-or-miss, but some people have at least gotten them to help with HG leaks out of warranty.
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Old 10-22-2011, 03:27 PM   #24
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My 07 warranty was 60k miles for powertrain.
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Old 07-18-2012, 11:10 PM   #25
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Quote:
Originally Posted by williaty View Post
OK, this one is coming back again because a couple of vendors offer what they call "O2 Sensor Spacers" that they incorrectly list as compatible with 2005+ Subarus. I'm getting PMs about these on a pretty regular basis. They look like this:



They're also available as spark plug anti-foulers or as little extension tubes welded on to the pipe.

Regardless of what they're called, what they do is to move the O2 sensor physically farther away from the main exhaust flow in hopes of tricking the ECU into not throwing the P0420 DTC when you run headers, a HFC, or a trackpipe. While on some cars they do actually prevent the P0420 DTC from being thrown, they cause fueling problems on all cars 2005 and newer. In 2005, Subaru switched to an ECU that uses the input from the rear O2 sensor to affect fueling in addition to simply monitoring the cat (as in 2004 and older cars). If you remove the rear O2 sensor entirely, mount it somewhere that isn't in the exhaust at all (don't laugh, I've seen it done), or even just move it out of the main exhaust flow, you'll develop strange fueling errors as the ECU tries to make the rear O2 see what it's programmed to achieve. The result of this is slightly worse fuel economy, a worse idle, and generally increased difficulty in getting the car tuned to a good state.

Summary: DO NOT USE O2 SENSOR SPACERS OR SPARK PLUG ANTI-FOULERS ON 2005 OR NEWER SUBARUS.


Also, it's worth noting that you can't use electronic "MIL Eliminators" like the ones from Pony Express on 2005 and newer Subarus either. Your only solution to getting a P0420 DTC on the newer cars is to disable it in the tune.
Not to sound like a jerk but why would the folks at Cobb say otherwise as far as the o2 function on the ecu and this is on a 2011 post so they are taking into consideration that this for a 2005+ car
Quote:
Originally Posted by COBB Tuning View Post
The rear o2 sensor's primary function is determining issues with the cat. It has very limited scope as far as A/F correction in normal operation. A high-flow cat or catless DP can cause the P0420 CEL because you are changing the characteristics of the exhaust flow around the sensor. This can cause the ECU to incorrectly believe there is a problem with the cat (too rich or too lean outside an acceptable range) and thus the CEL.

The Cobb AccessTUNER software can disable the P0420 code permanently. If you have a protuned map, get with your protuner to have it disabled. You will then need to reflash the map (not real-time as disabling DTCs must be done via a reflash).

Disabling the DTC is not going to cause ringland failure. For the 2.5L cars, you may run richer in closed loop if you remove the sensor from the exhaust or disconnect it.

Bill
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