downstream o2 and what it does

[-Alero-]

hey all, im about to go to the dealer (another) tommorow for an existing problem i have been having with P0420.

i need to convince them that the only purpose of the downstream O2 is that it reads to see if the cat is working.

i would really like hard evidence to possibly print out and bring to them saying "hey look the downstream O2 has nothing to do with how my car is going to idle."

im searching and searching for some hard facts/evidence to print out so i can take it to them. if anyone comes accross a website or an article explaining how the downstream O2 has no influence other than keeping the catalytic filtering properties in check, please supply me a link.

[CactusWill]

I've heard conflicting information on this one, some say it does others say it doesn't, so good luck because I'd like to know this myself.

[-Alero-]

yeah im pretty sure on these vehicles it is a strictly emissions monitoring device. if not i am mistaken

but its just every time i explain to a dealer what is going on with my car they just keep saying "you dont have the OEM cat on it thats why"

and they just dont want to believe a OBDII 50 state legal EPA approved cat from magnaflow, flowmaster or catco, "isnt up to par"

[jackal2000]

what about the fact that i don't have a down stream o2 and my car runs like a champ?

[-Alero-]

exactly jackal! the car is fine without it......1 example i have is my oxygen simulator i will bring in. ill explain how the car runs the exact same with a cat without a cat with this plugged in or not or downstream disabled.....of course it isnt written evidence.

but still something ill be sure to say

[Redog]

^^ Yeah you do that and see how quick they throw you out. :rolleyes2:
Downstream O2s help regulate the amount of fuel a car dumps into the engine. If the car's running rich the downstream sends a signal telling the PCM to back off on the fuel.

Too much fuel results in it being dumped into the exhaust valves and down into the exhaust. It gets burned up in the cat. Over time the cat becomes clogged with carbon and other crap.

A sim keeps a perfect conditon. Downstream O2 deletes only use the upstream O2s which is a better judge for the fuel curve anyway.

Downstream O2 is just for emissions

[-Alero-]

now redog you say the downstream is for emmissions. i completly agree with you on that.

but i always gathered that the upstream took care of all fuel and air ratios.

ive never followed the downstream wire but judging how it runs with or without 1 plugged in active or deactivated, it just seems less vital compared to the upstream.

[[ion] C2]

Lol misinformation.

Downstream is SOLELY for emissions (checks if the catalyst converts good or not).

Upstream is what controls closed loop fueling.

[-Alero-]

yes thank you ion, but i havent come accross hard evidence written down by GM or anything like that, i was hoping for something like that to print out. just cant seem to find what im looking for

[Cliff8928]

DTC P0420

Circuit Description

A three-way catalytic (TWC) converter controls emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that are present in the exhaust gas. This process converts the HC and the CO into water vapor and carbon dioxide (CO2), and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The powertrain control module (PCM) monitors this process by using a heated oxygen sensor (HO2S) that is in the exhaust stream after the TWC. This HO2S 2, also referred to as the catalyst monitor sensor, produces an output signal that the PCM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The PCM monitors the efficiency of the catalyst by allowing the catalyst to heat, then wait for a stabilization period while the engine is idling. The PCM then adds and removes fuel while monitoring the HO2S 2. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the response of the HO2S 1, which is located before the TWC. When the HO2S 2 response is near that of the HO2S 1, the oxygen storage capability and efficiency of the catalyst may be degraded below an acceptable threshold. If the PCM detects the degraded condition, DTC P0420 sets......

[-Alero-]

hmm nice post cliff but where di you get it from?

also this ite explains alot http://www.aa1car.com/library/p0420_dtc.htm

[Redog]

I was always under the assumption that the 2 O2's commucated with each other.

Now I'm not worried about the downstream O2 delete I want to get :D

[misslindseysue]

Downstream O2 also controls fueling, but not directly, on a longer feedback loop - it looks for longer fuel trim, whether you tend to run rich or lean. I don't know if/when GM started using downstream O2 for that, though, I just know about Chrysler and Hyundai/Kia OBD-II since that's what I've worked with.

[Redog]

I could be wrong but I think GM started using downstreams in 1996 when OBD-II came out.

With that but :offtopic: when does OBD III come out. I was told in 2004, still have OBD II. Not that I'm looking forward to it. I was told it will be a bitch to tune

[-Alero-]

Downstream O2 also controls fueling, but not directly, on a longer feedback loop - it looks for longer fuel trim, whether you tend to run rich or lean. I don't know if/when GM started using downstream O2 for that, though, I just know about Chrysler and Hyundai/Kia OBD-II since that's what I've worked with.


im just guessing butdepending on the car the O2 sensors will be doing different things. im more than confident all downstream Post cat O2 sensors are not strictly for emissions.

just the alero in this case

[-Alero-]

I could be wrong but I think GM started using downstreams in 1996 when OBD-II came out.

With that but :offtopic: when does OBD III come out. I was told in 2004, still have OBD II. Not that I'm looking forward to it. I was told it will be a bitch to tune


i believe obd 3 is out, and yeah its not the easiest to tune. but i know CAN seems to be just as popular as obd II now

[Redog]

When in doubt, Google is your friend ;)

http://www.asashop.org/autoinc/may/obd_iii_new.cfm

OBD III is not out yet and hasn't started testing, yet

If I had to put money on it, I'd say OBD III will come out with the government mandated 35 MPG cars in 2016.

Anyway, a dealership is NOT going to remove or alter a Downstream O2. I really don't think which dealership you go to will matter. In that respect, you are wasting your time

You could just get the PCM reprogramed with an DS O2 delete

[-Alero-]

oh thats news to me then, i thought it was out

oh and ken if you saying i should get the downstream delete, it wont fix my problem since i have the rough idle issue, that is giving the result of P0420.

it idles the same with a cat, without a cat, with a simulator in.....its pretty troublesome, and all the dealers around here just keep saying "you dont have an OEM cat thats why" and for some reason only think P0420 means "cats gone bad" and only that......its a real headache to get some so called ASE techs to actually diagnose and not just be so single sided : /

lol alero woes :(

[lvemy3100]

All of this discussion is making me lol

fact is on these cars the 2nd (after cat) o2 sensor does NOTHING other than monitor the exhaust gasses after the cat to let the PCM how the cat is working by comparing readings with the primary o2 sensor.

deleting the rear o2 sensor with a cat still installed will only set off the code in the PCM to let you know its not operating. if you change the DTC table properly the code wont throw but you wont have any kind of waring to let you know if your cat isnt working properly. this is not an issue if you also delete the cat but considering there is little to no HP to be gained from doing IMO its best to just run a cat and 2nd o2 sensor.

the primary o2 sensor is the ONLY o2 sensor that has anything to do with how the PCM calculates fueling needs...

having a code for a bad cat will not change how the car runs but the bad cat itself will.

the p0420 code you have will not be caused by a rough idle trust me.. if you dont listen to the vid of mfullers car idling with a cam.. no p0420 code there ;)

aftermarket cat can be thought of as being bad by the o2 readings the sensor gets even if the cat is perfectly fine. this means with and without the cat you can still get the code thrown

as for the rough idle start looking elsewhere but the p420 code is for sure because of the cat whether its good or bad.. no need to go to a dealer to find that out. you can either kill the code inthe PCM or you can do it with a sim either way will work.

[bdyman]

my car has the same problem same code, i have the ecotec, now my mechanic told me it could be a vacum leak,or a stuck fuel injector, iam going to him when the weather clears up here and when he fixes the problem, i will report back in here to give you a answer

[-Alero-]

All of this discussion is making me lol

fact is on these cars the 2nd (after cat) o2 sensor does NOTHING other than monitor the exhaust gasses after the cat to let the PCM how the cat is working by comparing readings with the primary o2 sensor.

deleting the rear o2 sensor with a cat still installed will only set off the code in the PCM to let you know its not operating. if you change the DTC table properly the code wont throw but you wont have any kind of waring to let you know if your cat isnt working properly. this is not an issue if you also delete the cat but considering there is little to no HP to be gained from doing IMO its best to just run a cat and 2nd o2 sensor.

the primary o2 sensor is the ONLY o2 sensor that has anything to do with how the PCM calculates fueling needs...

having a code for a bad cat will not change how the car runs but the bad cat itself will.



oh trust me i dont plan on deleting the code i dont have a problem with keeping the cat on, in no way shape or form am i trying to do that, i removed my cat a few years ago to see what it was like, ended up putting back on within a week.

i removed my cat again to prove to a local dealer my cat had nothing to do with my idling and they simply just said "its because you need an OEM cat".....Tons of facpalming that day

[-Alero-]

my car has the same problem same code, i have the ecotec, now my mechanic told me it could be a vacum leak,or a stuck fuel injector, iam going to him when the weather clears up here and when he fixes the problem, i will report back in here to give you a answer


yeah vacuum leak seems like it could be possible with me......its just playing the game of diagnosis is what the dealer doesnt want to do....its so lame...


my car is 90% stock now, stock airbox, stock pcm coils etc etc. all it has is the aftermarket exhaust, crank pulley and lowered......i only turned it to stock for the dealers to not label me as a "racer"

[bdyman]

according to my mechanic, if the cat is plugged or defective then yes replace i think we all know that one , but he said this problem is fixed without replacing the cat, as soon as mine is fixed i will tell ya what it is

[-Alero-]

according to my mechanic, if the cat is plugged or defective then yes replace, but he said this problem is fixed without replacing the cat, as soon as mine is fixed i will tell ya what it is


ok that works despite our different engines i would still like to know. im going to another dealer today with alot on my mind, ill be sure to post on what they say.....dreading having this arguement with a biased service writer.... sigh

[lvemy3100]

have someone hook up a scantool to your car and check individual cyl misfires and other parameters and see if you cant find your idle issue that way..

[bdyman]

yeah vacuum leak seems like it could be possible with me......its just playing the game of diagnosis is what the dealer doesnt want to do....its so lame...


my car is 90% stock now, stock airbox, stock pcm coils etc etc. all it has is the aftermarket exhaust, crank pulley and lowered......i only turned it to stock for the dealers to not label me as a "racer"


dealership tech are parts changers ,plug in the scan tool ohh its this part, unless your willing to pay out the ass for their work your alomost better off going to a ma and pop shop thats where i go hes updated on the newer cars gos to the gm training

like next week on a nice sunny day iam going to him as soon as iam back i will tell ya

[bdyman]

have someone hook up a scantool to your car and check individual cyl misfires and other parameters and see if you cant find your idle issue that way..


yes like check fule injectors pulse, and some other shit he was going to do, check for vacum leaks, btw arent you a mechanic??

[lvemy3100]

yes like check fule injectors pulse, and some other shit he was going to do, check for vacum leaks, btw arent you a mechanic??

indeed I am.. and happy to help out almost anyone on this forum if they can make it to Round Lake, IL ;)

[-Alero-]

i would seriously kill for MPracing or WOT to look at my car. but yeah its just, i think we have lazy techs here or something because none of them want to use thier scan tool.... they just pull the code and go from there....so lame

[lvemy3100]

i would seriously kill for MPracing or WOT to look at my car. but yeah its just, i think we have lazy techs here or something because none of them want to use thier scan tool.... they just pull the code and go from there....so lame

Idk where you are located but if you can make it out here I am happy to check it out..

kinda weird that they dont wanna do any more than pull the code since out here the first thing they (dealer techs) try to do is tell people that they need to hook up the scan tool and then try to charge an minimum of an hr labor @$120/hr that no one can afford anymore. only reason i know is cause I end up with alot of them coming to me for the free 5min scans I offer (if they are lucky enough to hear about me before being ripped off)

[[ion] C2]

Long term fuel trims are not what the downstream O2 is for. The computer starts to make long term fuel trims based on the sheer amount of the same values for the short term fuel trims. If there's a lot of -5% going on all the time, that's going to be logged as long term eventually since it's obviously not a short term fueling change.

Rear O2 is solely for catalyst efficiency monitoring and has nothing to do with fuel control in closed loop operation, as Cliff8928's post clarified.

[bdyman]

indeed I am.. and happy to help out almost anyone on this forum if they can make it to Round Lake, IL ;)

dude i would like to but thats just way to far for me to travel, you could come to me if ya want........

[Cliff8928]

hmm nice post cliff but where di you get it from?

also this ite explains alot http://www.aa1car.com/library/p0420_dtc.htm

factory service info.

[Raziel]

I don't know if it would make much sense, but have you checked your IAC modual or motor, I had a prob with my Alero with starting and idling rough and that is what it turned out to be. Maybe try that and see what happens.

Raz

[-Alero-]

idle air control valve gets cleaned regularly, havent changed it but seems to be in working order....

got back from dealer, no help, they said all my sensors read just find and that i need a new catalytic converter.....just like all the others

they also said i had nothing wrong with my idle, and said they have no reason to perform diagnosis on an engine they believe nothing is wrong.

i told them over and over how little influence the cat has on my idle and i can come in with a hollow cat no cat, disabled downstream O2 sim, and it will still run the same.

they said it needs a OEM cat and nothing in the world is better than a OEM cat. i then asked why would companies produce OBDII spec 50 state legal EPA approved converters, and he said they are no where near oem spec.

i then got in a very very heated arguement with the service writer/manager who took a ride with me, and the last thing i said was "im not going to play the game of buying your 900$ cat and then come in a week later with the same code and the same problem" he then said "well then this game is over" and got out.....

end of story, back to square one......no idea what to do.... it idles like garbage and im told it idles flawlessly hwen you stare at the engine it bucks back in forth like its having the time of its life....

FML

[jackal2000]

have you checked the motor mounts?

[-Alero-]

have not looked at the very rear one near firewall. the side one near pullies seems fine, and the front tranny mount seems just fine.

pushing weight in different directions on the engine while its off, seems fairly solid....

but if i took this thing to ASS and let any one of you sit in it i guarentee every single one of you would say "whats wrong with your idle?".......

sigh might cut my losses i dunno.....dont have the money to replace the car though so im basically in a sweet spot

[[ion] C2]

take it to ass dude it's easier to diagnose a problem when you have 24 alero guys on site

[-Alero-]

the car is basically stock now i dont have much to show anymore...... jackal actually lives relatively close he has a program ive been wanting to use for a while....

[[ion] C2]

not that excuse again. zoL's coming from massachusetts, so is nate's alero from NH and they're basically stock.

hell 3 california guys are coming, a pretty close to stock, why not come check out everyone's aleros?

guiguilandry with his 3800 s/c swap, custom hood, custom trunk, custom interior
rowdygls with his all around modified car
bdyman's work of art
etc.

[-Alero-]

yeah yeah yeah. with my alero headache being around more isnt something thats gonna help :(

[Redog]

Jackel's in the middle east, so you'll be waiting a while

[TurboAleroGT40]

Hey dudes, I know this is a old thread.
But my question is, where is the connector for the rear O2 sensor located. On which harness is it on? because I removed it when I went turbo. Now I dont remember where it was because I think I taped it into the harness it self.

[Cliff8928]

It's not too far from the rear trans mount.

[TurboAleroGT40]

Hmmm, well searched for that damn O2 connector but with no luck. On which harness is it located? On the engine harness or chassis ?

[Redog]

IDK.

On the ground just get the car up high enough and reach up into the engine bay right next to the downpipe. It will have a clip holding the plug into the hardness

[TurboAleroGT40]

IDK.

On the ground just get the car up high enough and reach up into the engine bay right next to the downpipe. It will have a clip holding the plug into the hardness


The O2 sensor plug was cut and hidden in the harness.
So now i'm searching for it and I dont remember where it was. So was it on the chassis or engine harness???