Rear O2 sensor Important!?

[styx0r]

Hi,

 

I keep reading that the rear o2 sensor is used to check cat efficiency. I am running entirely catless, so the rear o2 sensor should be freaking out. It isn't. I'm pretty sure my PDX stage 2 map is designed to ignore the rear o2 code.

 

My question is, do I need this sensor, or can I remove it entirely?

 

I appreciate your input!

[whitetiger]

your tune takes car of the codes cause by the sensor not seeing cats inthe exhaust. technically you dont need it but since the comuter needs to see somthing plugged in there, you should keep it. its not hurting you to keep it.

[styx0r]

Wait, so you're telling me: I technically don't need it - and then you're saying the car needs to see something plugged in there, which implies that I DO need it.

 

The reason I am asking this question is because my rear o2 sensor wire is damaged (although the sensor seems to work). I have the option of placing a bolt in the hole that the sensor normally fills.

 

If this will not freak out the ECU or cause a CEL, I will be doing this.

[boostsr20]

f you remove it you will probably get a CE light. When mine came unplugged I was getting a CE light for "starter switch low input" P1518. Not sure how its related but as soon as I plugged it back in, the CE light went away.

[itsme]

You need the rear sensor for fuel trim learnings. I think it's called. I took my factory rear O2 out and replaced it with a wide band O2 senor to do engine tuning and my car ran very rich. My Air /fuel was showing @10.7 at idle instead of the @14.7 it was suppose to show. From what I read even though you get rid of the rear O2 CEL it still send info to the ECU so it can adjust the fuel mix throught the entire range. After I put another bung for the WBo2 and reinstalled the factory o2 it went up to 14.3(I have a K&N typhoon and the MAF is a little off.) I would reinstall it if I was you or contact some of the self tunners for more info.

[styx0r]

itsme - Thanks man! That was exactly what I was unsure of. This thread will hopefully help others in the future as well.

[whitetiger]

You need the rear sensor for fuel trim learnings. I think it's called. I took my factory rear O2 out and replaced it with a wide band O2 senor to do engine tuning and my car ran very rich. My Air /fuel was showing @10.7 at idle instead of the @14.7 it was suppose to show. From what I read even though you get rid of the rear O2 CEL it still send info to the ECU so it can adjust the fuel mix throught the entire range. After I put another bung for the WBo2 and reinstalled the factory o2 it went up to 14.3(I have a K&N typhoon and the MAF is a little off.) I would reinstall it if I was you or contact some of the self tunners for more info.

 

no the rear sensor is not used for fuel trim learnings. thats what the front sensor is for. the rear sensor is just there to check cat efficiancy. If you can ensure that the codes for both the sensor and heater circuit for the rear sensor dont come up by having them disabled with the accessport, then you dont need the rear sensor at all. Im not sure that your average stage 2 base map get rid of the heater circuit codes, just cat efficiancy codes. you would have to have a tuner confirm this.

[whitetiger]

Wait, so you're telling me: I technically don't need it - and then you're saying the car needs to see something plugged in there, which implies that I DO need it.

 

The reason I am asking this question is because my rear o2 sensor wire is damaged (although the sensor seems to work). I have the option of placing a bolt in the hole that the sensor normally fills.

 

If this will not freak out the ECU or cause a CEL, I will be doing this.

 

 

why dont you just fix the wire. its very easy to do.

[itsme]

I use enginuity to tune my car and I have the rear o2 CELs turned off. All that I'm talk about what I have seen. If you data log your car without a rear O2 where does your car get it fuel trim learnings from?

[whitetiger]

from the front one. how can the car get accurate readings from a sensor after the cats. the front one is for fuel trims, the rear one ensures the cats are working tey way they should.

[Spec B]

My 2 cents..I damaged the wire on my rear o2 and even with a stage 2 tune I got a Cel.

 

I cleared it a few times and it kept coming back...car ran fine tho. I ended up splicing the wire back on...no problems since.

[itsme]

from the front one. how can the car get accurate readings from a sensor after the cats. the front one is for fuel trims, the rear one ensures the cats are working tey way they should.

I don't have a front O2 with my stage 2 setup and both of my rear O2s are before my 3" hi flow catt. You still have a front O2 with your stage 2 setup? You are the first peson I know of that kept it.

[whitetiger]

^the front o2 is in the headders before the turbo. the car absolutly needs an o2 ther to measure combustion for closed loop fuel trim. I thnk you are forgetting some things.

[bugblatterbeast]

no the rear sensor is not used for fuel trim learnings. thats what the front sensor is for. the rear sensor is just there to check cat efficiancy. If you can ensure that the codes for both the sensor and heater circuit for the rear sensor dont come up by having them disabled with the accessport, then you dont need the rear sensor at all. Im not sure that your average stage 2 base map get rid of the heater circuit codes, just cat efficiancy codes. you would have to have a tuner confirm this.

 

do we know for sure that this is the case? some fuel injection systems (Bosch for example) use the front O2 sensor for rapid and large fuel trims but also have a secondary slower feedback loop driven by the second sensor (see page 667 of the Bosch Automotive Handbook, 6th edition).

[johnboi85]

i am highly skeptical about the rear o2 sensor having anything to do w/ the fuel trims. say it "does" tweak the fuel trims, how is the o2 sensor getting accurate data w/ people running catless?

[itsme]

I may be wrong about the fuel trims but my car ran richer without it.

[bugblatterbeast]

i am highly skeptical about the rear o2 sensor having anything to do w/ the fuel trims. say it "does" tweak the fuel trims, how is the o2 sensor getting accurate data w/ people running catless?

 

I'm not claiming it does on our cars. I was just commenting that there are some FI systems which use the second sensor to provide additional trimming. Merchgod could probably tell us for sure if there is any mixture control feedback from the second sensor.

 

Usually the second O2 is of the narrow band variety. When it is used after a cat it there to 1) check for cat efficiency 2) make sure the mean mixture through the cat is stochiometric and 3) that the deviation rich or lean doesn't last long enough to deplete the stored oxygen or saturate the convertor with oxygen. Even catless, 2 and 3 can still be done as the sensor is only looking for residual oxygen inside the pipe or outside the pipe (atmoshpheric reference).

[merchgod]

I'm not claiming it does on our cars. I was just commenting that there are some FI systems which use the second sensor to provide additional trimming. Merchgod could probably tell us for sure if there is any mixture control feedback from the second sensor.

The SSM parameters "A/F Correction #3" and "A/F Learning #3" would indicate that there is. The factory manual also says the following - "The air/fuel (A/F) control system makes a correction to the basic fuel injection duration in accordance with the signals from the front oxygen (A/F) sensor and the rear oxygen sensor so that stoichiometric ratio is maintained." Problem is, while I can directly trace the impact of A/F correction #1 and A/F learning #1 (i.e. front o2) on the base pulse width, I have not been able to do the same for the rear o2 (#3). It is possible to limit A/F corr/learn #3, but the values are hard coded and not a traditional min/max (large range of -50%/+50%). I suspect that this is because there is a potentially large possible variation (removing the rear o2 is known to cause a rich condition). I think in normal operation, the actual correction is small. One thing, which is a problem, is that the current conversion for A/F Correction #3 is incorrect and results in wacky values if you attempt to log it. I'll fix this in the next logger update.

 

If there's someone who wishes to try a test fix (at your own risk) of eliminating rear o2 correction/learning, PM me at the romraider site.

[whitetiger]

do we know for sure that this is the case? some fuel injection systems (Bosch for example) use the front O2 sensor for rapid and large fuel trims but also have a secondary slower feedback loop driven by the second sensor (see page 667 of the Bosch Automotive Handbook, 6th edition).

 

 

no this is not true of subaru. subaru only uses the front sensor for trims. the rear senson ONLY checks cat effciancy.

[merchgod]

no this is not true of subaru. subaru only uses the front sensor for trims. the rear senson ONLY checks cat effciancy.

Source?

[whitetiger]

no source i can list. Logic would only indicate that the rear o2, being after 2 cats on the stock car would not be able to give any viable tunning info to the ecu anyways especailly seeing how far down stream the rear sensor is. i cant possibly see how it would affect tuning if removed. Even stage 2 maps disregard that consor completely anyways from what i understand. I dont see how there being a 3rd set of A/F tuning perameters in the ecu would indicate that the sensor affect tuning. And the fact that removing the sensor causes a rich condition probably occurs because people also remove the cats when they take the sensor out, and when not tuned properly for no cats, the car runs richer anyways due to increased exhaust flow and the effects that has on the car's factory tuning.

[merchgod]

no source i can list. Logic would only indicate that the rear o2, being after 2 cats on the stock car would not be able to give any viable tunning info to the ecu anyways especailly seeing how far down stream the rear sensor is. i cant possibly see how it would affect tuning if removed. Even stage 2 maps disregard that consor completely anyways from what i understand. I dont see how there being a 3rd set of A/F tuning perameters in the ecu would indicate that the sensor affect tuning. And the fact that removing the sensor causes a rich condition probably occurs because people also remove the cats when they take the sensor out, and when not tuned properly for no cats, the car runs richer anyways due to increased exhaust flow and the effects that has on the car's factory tuning.

Well, this directly contradicts the factory manual. Plus the numbered parameters indicate the sensor (#1 = o2 sensor 1 (front), #2 = o2 sensor 2 (front - only the H6 Subarus have the second front o2 sensor), #3 = o2 sensor 3 (rear)), not an ordered list of corrections. It is well known that removing the rear o2 completely (or leaving it connected but pulled from the exhaust) results in a rich condition in closed loop with the 32-bit ECU. Now, that doesn't prove that the rear o2 is involved normal fuel trims as it could just be based on a perceived lean condition to preserve the cat.

 

Stage 2 maps do not remove the rear o2 sensor from any influence - typically, the CEL would be disabled and the threshold pertaining to cat efficiency may be changed (pertaining to obdii readiness status) but this does not change any existing a/f correction/learning.

[whitetiger]

Now, that doesn't prove that the rear o2 is involved normal fuel trims as it could just be based on a perceived lean condition to preserve the cat.

 

this is what I think the case is.