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Resolved front/rear O2 sensors - A/F while driving - emissions

ErikTande

Supporting Member
1,446
443
Dec 17, 2012
Centennial, Colorado
Which o2 sensor is responsible for keeping track of the air fuel ratio while driving? Front, rear or both?

I failed emissions with really high CO GPM, which means I was running very rich.

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Car is bone stock and passed emissions 2 years and only 1000 miles ago (car sat most of the 2 years).

I did drop the entire exhaust recently, including removing and re-installing the rear O2 sensor. Maybe I damaged it.

I'm going to:
Boost leak test
sea foam the shit out of it
replace o2 sensors (should I just do both to be sure?)
add 1 gallon of e-85 to a full tank

if that doesn't work,
add an air pump to pump clean air into the exhaust
(lots of cars do this from factory)
 

95REGF150

Proven Member
433
218
Mar 7, 2012
Elk Grove, California
The front O2 is the only one that controls the engine fuel trims.

The rear O2 is simply there for catalytic converter performance check and triggers the P0420 CEL Code if it's signal too closely matches the front O2 signal indicating the catalytic converter is not doing its job.

i.e. The ecu monitors both sensors and checks the rear O2 against the front o2 during cruise and idle. Properly functioning catalytic converters have capacity for storing oxygen. So when the ECU oscillates the mixture from rich to lean and back again while in closed loop based on on the input from the front O2 a properly functioning cat will slow this oscillation when measured by the rear O2. When the 2 signals become too close or the same based on a set level by the manufacturer the ECU will trigger a P0420 code. Just because your car did not trigger a P0420 code does not mean that the cat is working at it's highest efficiency.

I don't think that your own rigged up secondary air system will get you very far. Its certainly not going to be the simplest solution to the problem. Also I think adding E85 without a way to tune or at least a wideband to monitor A/F is a dangerous way to go. I did notice however that while both of these things were included in your list of "fixes" replacing the catalytic converter was not.

The Cats primary job is to reduce HC & CO emissions so if your high on those and you find the car is not running rich due to a bad front O2 then that is where I would look. The up side is you have good NOX numbers which are the hardest to get in check. HC & CO are either too rich or bad cat pretty simple really.
 
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ErikTande

Supporting Member
1,446
443
Dec 17, 2012
Centennial, Colorado
The front O2 is the only one that controls the engine fuel trims.

The rear O2 is simply there for catalytic converter performance check and triggers the P0420 CEL Code if it's signal too closely matches the front O2 signal indicating the catalytic converter is not doing its job.

i.e. The ecu monitors both sensors and checks the rear O2 against the front o2 during cruise and idle. Properly functioning catalytic converters have capacity for storing oxygen. So when the ECU oscillates the mixture from rich to lean and back again based while in closed loop based on on the input from the front O2 a properly functioning cat will slow this oscillation when measured by the rear O2. When the 2 signals become too close or the same based on a set level by the manufacturer the ECU will trigger a P0420 code. Just because your car did not trigger a P0420 code does not mean that the cat is working at it's highest efficiency.

I don't think that your own rigged up secondary air system will get you very far. Its certainly not going to be the simplest solution to the problem. Also I think adding E85 without a way to tune or at least a wideband to monitor A/F is a dangerous way to go. I did notice however that while both of these things were included in your list of "fixes" replacing the catalytic converter was not.

The Cats primary job is to reduce HC & CO emissions so if your high on those and you find the car is not running rich due to a bad front O2 then that is where I would look. The up side is you have good NOX numbers which are the hardest to get in check. HC & CO are either too rich or bad cat pretty simple really.


Thanks! I'm assuming the CAT is good considering it passed 2 years ago, and only 1000 miles have been put on the car since then.

So I should just replace the front O2 for now, right? That's the one controlling fuel trims?

I'll be removing the rear O2 sensor after I pass emissions anyway (new catless exhaust).
 

95REGF150

Proven Member
433
218
Mar 7, 2012
Elk Grove, California
I know I've looked through your 1G build thread before and it had wideband and the like so I am assuming this is on your 2G correct?

If you get a cheap OBD scanner dongle from Amazon that bluetooth to your phone you can monitor the front O2 sensor voltage. Should sit around 0.5 during cruise with the oscillation above then below 0.5v. If the voltage is stuck below 0.5 like say down to 0.1 then it is constantly telling the engine its lean which will turn the car rich if its an incorrect reading. Should kick a lean condition or O2 sensor malfunction/low voltage code if that was the case.

Or just throw an O2 sensor at it and go get another test. I'm just one of those guys that likes identifying the problem over doing the old parts slap game. Usually costs less in money but more in time in the end.
 

ErikTande

Supporting Member
1,446
443
Dec 17, 2012
Centennial, Colorado
Yeah it's my 2g. I have one of those scanners, I'll try that and report back!

Should i have any voltage at idle? Cuz i don't. I didn't take it for a cruise yet.

I have .1v on the rear and 0.0v on the front.

It is plugged in, I just checked. I'll unplug, clean and re-seat the connection and see if I get voltage later tonight.

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95REGF150

Proven Member
433
218
Mar 7, 2012
Elk Grove, California
If the ignition is on you should have voltage. If not running it should be closer to 0.1 though indicating lean. If idling it should be near 0.5v and oscillating back and forth above and below it. Like 0.3v - 0.7v or somewhere in there and back and forth. On a narrowband sensor 0.5v = 14.7:1 AFR. In closed loop it is always trying to hit 0.5v but does not have a control system to be able to do that. So it constantly overshoots over and then back under creating the oscillation. If it is too far over or under the 0.5v during the oscillation it will add or subtract from the long term fuel trims accordingly to bring it in closer.

Here is a narrowband O2 sensor graph to make it easier to visualize. The reason this works is the sensor is not linear like our wideband outputs. You can see from 0.7-0.3 is pretty much 14.7 so even though its oscilating in voltage that much it keeps it right at 14.7. When it drops to 0.1 though it takes a exponential turn lean and vice versa rich toward 1 volt.

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Also shows why narrowbands are useless for tuning WOT. They are a sensor designed to measure one specific AFR at 14.7:1 and have no accuracy anywhere else in the range. So if you want to tune your WOT pull in boost at 11.5 AFR lets say. Then you are SOL looking at the narrowband voltage.

Some of the OBD scanners are incompatible with our cars. Do you have other information coming from it like the Engine Speed, etc.? No sense trying to track down lack of voltage if the scanner is not working.

If the scanner is working then looks like you may indeed have a bad sensor. Just not sure why it didn't kick a code for that.
 
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ErikTande

Supporting Member
1,446
443
Dec 17, 2012
Centennial, Colorado
Ok, I disconnected my front o2, cleaned the connectors with electrical cleaner and reconnected them.

I verified my scanner is working properly and I added the correct sensors. If I add a sensor my car doesn't have, I don't get a green light by it. So all these sensors (attached) are correct.


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I went for a drive and my front O2 sensor (o2 1x1 in the app) pretty much stayed 0.0, it did flash 0.3 a couple times but I could never get the screenshot. My rear O2 sensor (o2 1x2 in the app) never fluctuated much, mostly stayed around 0.1, to 0.3.

I don't have any check engine lights.

So I ordered a new front o2 sensor, it'll be here Friday. I'll update the thread when I get it installed.







**EDIT** I just realized I can have the app automatically hide unsupported sensors. I did that and still have those 2 voltage readings, no change though. still 0.0
 
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