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Old 10-21-2012, 12:42 AM   #1
MRF22
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My 05 STi currently has no cat and no rear O2 sensor. I just got it tuned due to adding some bolt ons and I was curious if I put a rear O2 sensor in will it mess with the tune? I have heard bad things about not having your rear O2 sensor and A/F ratios being messed up. Will I need a new tune if I put in the rear O2 sensor?
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Last edited by MRF22; 10-21-2012 at 01:30 AM.
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Old 10-21-2012, 10:20 AM   #2
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Front O2 sensor is AFR and is important for the ecu, rear is emissions related.
I run a cobb catted downpipe since I don't want the environment to smell like Mexico City air quality and kept the rear O2 sensor.
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Old 10-21-2012, 11:56 AM   #3
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Quote:
Originally Posted by Philbertio View Post
Front O2 sensor is AFR and is important for the ecu, rear is emissions related.
The presence/absence of the rear O2 does affect the closed loop fueling target on the 32-bit ECU though.
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Old 10-21-2012, 12:34 PM   #4
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Quote:
Originally Posted by the suicidal eggroll

The presence/absence of the rear O2 does affect the closed loop fueling target on the 32-bit ECU though.
So if I am tuned without the rear O2 sensor would I have to get a new tune if I put my Rear O2 sensor in?
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Old 10-21-2012, 01:06 PM   #5
the suicidal eggroll
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I'm not sure
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Old 10-21-2012, 01:08 PM   #6
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Quote:
Originally Posted by MRF22

So if I am tuned without the rear O2 sensor would I have to get a new tune if I put my Rear O2 sensor in?
Call your tuner
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Old 10-21-2012, 04:57 PM   #7
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I called and asked my tuner and he said that the rear O2 sensor is only there to make sure the cat is working.

I have heard horror stories of not running a rear O2 and that gas mileage will go up if I run be and my at will run quite better.
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Old 10-21-2012, 07:08 PM   #8
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Doesn't the lack of rear o2 cause the car to run richer? I had an open dump pretty much from the turbo pointing towards the ground (no rear o2) and the car ran richer. Cruising was normal.
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Old 10-22-2012, 04:52 PM   #9
MRF22
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Ok so it shouldn't affect my tune.

Now I have another question.
If I put a high flow cat on my car will it need a re-tune?
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Old 10-22-2012, 05:28 PM   #10
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if you are tuned on opensource you need the rear sensor on the 32bit cars as suicidal said. if you are tuned on AP there is a way around this.

so which are you tuned on?

Wicked M@
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Old 10-22-2012, 05:39 PM   #11
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just get a wideband and run its sensor in the rear o2 bung.
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Old 10-22-2012, 06:01 PM   #12
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I have an open source. My tuner said that the rear O2 sensor is not needed. I have one and will put it in. I don't have any CEL's and no cat. I will be getting the Turbo XS high flow cat soon.

I don't know what wideband is. I have heard of it but never been told what it is.
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Old 10-22-2012, 06:28 PM   #13
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Id be skeptical unless he hex edited the map to negate the effects of AF Correction 3. Turning off the CEL doesn't change the CL fueling effects of not having the rear 02 connected.
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Old 10-22-2012, 06:32 PM   #14
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on 16 bit cars, this is not the case. and the ability to negate this compensation is NOT built into ECUFlash or Romraider, which are the 2 most popular editing platforms.

running a wideband in the rear o2 spot wouldnt help this issue at all.
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Old 10-22-2012, 06:42 PM   #15
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Quote:
Originally Posted by Wicked Innovation
on 16 bit cars, this is not the case. and the ability to negate this compensation is NOT built into ECUFlash or Romraider, which are the 2 most popular editing platforms.

running a wideband in the rear o2 spot wouldnt help this issue at all.
Ok so I def need to get that O2 sensor in ASAP. My tuner said the rear O2 sensor is only used for monitoring the cat and that it doesn't communicate with the ECU.
So just to confirm I will be ok with mg tune if I install the O2 sensor?
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Old 10-22-2012, 06:49 PM   #16
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I would assume yes, but I cant answer that for sure.

The statement that the rear o2 doesnt communicate with the ecu is false. It does and when there is a fault it skews the value you can log as AF Correction 3. This will change your closed loop fueling target.

It is possible to change in the tune directly with a hex editor, or if your tuner made custom defs he uses to edit maps.
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Old 10-22-2012, 07:04 PM   #17
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Quote:
Originally Posted by Wicked Innovation
I would assume yes, but I cant answer that for sure.

The statement that the rear o2 doesnt communicate with the ecu is false. It does and when there is a fault it skews the value you can log as AF Correction 3. This will change your closed loop fueling target.

It is possible to change in the tune directly with a hex editor, or if your tuner made custom defs he uses to edit maps.
So if he made those changes in the tune would it hurt of help my car if I put it in?
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Old 10-22-2012, 08:06 PM   #18
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If the min/max ranges of that correction were zeroed, adding a rear 02 would make no change.

FWIW, Ive never seen a car in the wild with this done properly on opensource.

M@
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Old 10-22-2012, 08:54 PM   #19
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Quote:
Originally Posted by Wicked Innovation
If the min/max ranges of that correction were zeroed, adding a rear 02 would make no change.

FWIW, Ive never seen a car in the wild with this done properly on opensource.

M@
Ok thanks for the info. I have heard so many people and tuners say that the car will run much better and obtain better fuel economy with the rear O2 sensor in. But at the same time I have been told that its pointless. Lol I'm gonna throw it in and see what happens.
Thanks again.
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Old 10-23-2012, 10:59 AM   #20
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Quote:
Originally Posted by Wicked Innovation View Post
If the min/max ranges of that correction were zeroed, adding a rear 02 would make no change.

FWIW, Ive never seen a car in the wild with this done properly on opensource.

M@
Well the definitions exist now for some cars. Downloaded them today from the hyper experimental dont use these if you dont know what you're doing section of the RR Github. Even without the deffs you could still disable it with a hex editor.
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Old 10-23-2012, 11:51 AM   #21
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Fair enough. Still though, most people arent very on top of the latest and greatest random defs. Ive added this functionality into the basemaps I use myself.

I would be inclined to believe based on the responses the OP posted that his tune does not have this configured correctly.
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Old 10-23-2012, 01:26 PM   #22
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On the 08+ cars, the ECU does use the rear O2 sensor to adjust fueling. It is limited to +/- 2.5% but this in enough to cause the car to run rich at an idle and not hit the desired AFR at WOT. If you pull your rear O2 you will see the idle go rich. Not to mention it will throw off your long term fuel trims affecting fueling in other areas as well.

The newest Cobb ATR now has the ability to limit the rear O2's affect on fuel trims to 0% so that presumably you can just ditch the rear O2 sensor now. This update just came out this month. Otherwise the ONLY thing that was tuned out of 08+ cars regarding the rear O2 was telling the ECU to NOT throw a CEL for rear cat efficiency. However, it left the functionality of the rear O2 in place.

Note: I assume the behavior is the same for all 32 bit ECU's but I only have experience with 08+ ECU's when it comes to Subaru.
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Old 10-23-2012, 01:29 PM   #23
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Quote:
Originally Posted by MRF22 View Post
Ok so it shouldn't affect my tune.

Now I have another question.
If I put a high flow cat on my car will it need a re-tune?
No you wont need another tune for a hi flow cat. As for the Rear O2, you might want to read my post right above this one. ^^^
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Old 10-23-2012, 04:07 PM   #24
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Quote:
Originally Posted by Tripintaz

No you wont need another tune for a hi flow cat. As for the Rear O2, you might want to read my post right above this one. ^^^
Ok so my tuner said he tuned it so the rear O2 sensor wasn't at play. He seems to believe that the only thing the rear is used for is making sure the cat works. I'm so confused.
So should I put the rear O2 sensor in?
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Old 10-23-2012, 04:37 PM   #25
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Tripintaz, its the same for all 32 bit.

MRF22. Well he is wrong, so if he doesnt know how the rear o2 works, how did he tune it to not influence anything? lol.

If you have logging ability log AF Correction 3 (32 bit) and see if it has a value other than 0.

M@
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