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.
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]

Originally Posted by GM

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

[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 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-]

Originally Posted by 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.

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-]

Originally Posted by Redog

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

With that but 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