Downstream O2's

[Lazzman]

Hello,

I have an idea of what the down stream O2 sensors on my '98 soch V6 do, I know that they monitor the Cats for efficiency and proper working order- but can they actually effect the performance of a vehicle?

What if they were totaly carboned up and were not sending a signal a rich or lean signal at all, what would happen?

I have not replaced the downstream O2's since it was purchased new,that was 8yrs & 160k hard miles ago.

I have no check engine light, do I really need to replace them?? I did do the fronts at 125k mi on the advice of a mechanic and I did see about a 15% increase in gas milage.

I ask because from what I have read the down stream O2's only monitor the Cats function and cannot richen or lean the mixture as the upstream O2's can.

Somebody please help.

Thanks

 

[MoabB2]

Hello,

I ask because from what I have read the down stream O2's only monitor the Cats function and cannot richen or lean the mixture as the upstream O2's can.

Somebody please help.

Thanks

That is my understanding as well. I don't think they can affect performance.

 

[stup]

the post cat o2 sensor does not affect performance. it is only there to monitor oxygen leaving the catalyst. however, if they are severely fouled to the point where they do not read correctly, the engine computer will know this and will throw a code. and since it is an emissions related part, it can/will turn on the light. so basically, if the light isnt on, you should be fine.

 

[aldive]

Easy solution is to get a computer tune to turn off those cats.

Edit: I didn't see that it was Lazz who posted first. You have a chiop, the rear sensors should be turned off already.

 

[Lazzman]

Yeah, but I don't think so. The guy who owns the dyno shop indicated that the 02's were still functioning and reading the a/f ratio, don't know if he was referring to the downstreams though.

Also when I threw a code recently the scanner picked up Bank 1 position 2 lean.

Oh well screw it, I don't wanna spend no more on this truck.

 

[stup]

Yeah, but I don't think so. The guy who owns the dyno shop indicated that the 02's were still functioning and reading the a/f ratio, don't know if he was referring to the downstreams though.

Also when I threw a code recently the scanner picked up Bank 1 position 2 lean.

Oh well screw it, I don't wanna spend no more on this truck.

bank 1 pos 2 would be the post cat, so maybe they do need to be replaced.

 

[Lazzman]

I threw the code for another issue, I just thought it was interesting that scanner said one of the downstream injectors was lean.

With this in mind, I read on an information page about O2 sensors, that if the downstreams read lean they will send a signal to the ecm, which will in turn richen up the mixture until the downstream 02 is satisfied- if one of the downstream 02's is fouled it will continously read a lean condition causing the ECM to richen the mixture on a continous basis. Does this seem correct?

Al, it would be beyond the legal ethics of a licensed tuner in the state of Massachusetts to purposely disable the rear 02sensors. If the cats mulfunction and are no longer doing their job, the vehicle would surely fail emissions. The only way to tell if this is true is to have the downstream Oxygen sensors in working order.

 

[aldive]

Oh well screw it, I don't wanna spend no more on this truck.

Does that mean no dyno?

 

[aldive]

Al, it would be beyond the legal ethics of a licensed tuner in the state of Massachusetts to purposely disable the rear 02sensors. If the cats mulfunction and are no longer doing their job, the vehicle would surely fail emissions. The only way to tell if this is true is to have the downstream Oxygen sensors in working order.

What is a licensed tuner?

 

[gijoecam]

Found some interesting semi-related info on them...

Catalyst Efficiency Monitor - Federal Test Procedure
The Federal Test Procedure Catalyst Monitor monitors the catalyst system for deterioration and illuminates the MIL when tailpipe emissions exceed the appropriate HC emission thresholds. It is called the FTP catalyst monitor because it must complete during a standard emission test (the Federal Test Procedure). This monitor relies on the front and rear heated oxygen sensors (HO2S) to infer catalyst efficiency based upon oxygen storage capacity. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the rear HO2S quite slow compared with the frequency of the front HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines, and the rear HO2S begins to switch more rapidly, approaching the frequency of the front sensor. In general, as catalyst efficiency decreases, the switch ratio increases from a switch ratio of 0 for a low mileage catalyst to a switch ratio of 0.8 or 0.9 for a low-efficiency catalyst.

Many Low Emission California vehicles will monitor substantially less than the entire catalyst volume in order to meet the stringent catalyst monitoring malfunction thresholds. In many cases, only the front, light-off catalyst is monitored.

Front and rear HO2S switches are counted under specified closed loop fuel conditions. After the required number of front switches are obtained, a rear-to-front HO2S switch ratio is calculated. The switch ratio is compared against a threshold value. If the switch ratio is greater than the emission threshold, the catalyst has failed. Inputs from the ECT or CHT (warmed up engine), IAT (not at extreme ambient temperatures), MAF (greater than minimum engine load), VSS (within vehicle speed window) and TP(at part throttle) are required to enable the Catalyst Efficiency Monitor.
The DTCs associated with this test are DTC P0420 (Bank 1) and P0430 (Bank 2). Because an Exponentially Weighted Moving Average is used for malfunction determination, up to six driving cycles may be required to illuminate the MIL.

Couldn't find any specific mention of the rear sensors' function, although one could infer from this information that their sole purpose is to monitor the catalyst efficiency, and they only function relative to the upstream sensors.

-Joe

 

[Lazzman]

Yes,I agree Joe.

I ment to say licensed business in Massachusetts, obviously a registered business cannot perform illegal acts like disabling emissions devices. It is kind of like the window tint issue in MA, 95% of all shops will not tint outside the legal limit of 35% light blocking for fear of legal action.

The dyno tune is still on if the damn dyno shop would ever call me back and let me know when to come in.

Al, I will also be haveing the gap on my plugs widened to .065 to accomodate my Screamin Deamon Coil.

 

[aldive]

The dyno tune is still on if the damn dyno shop would ever call me back and let me know when to come in.

I have never heard of a dyno shop that is too busy to schedule a dyno run(s).

 

[Lazzman]

No, no not the shop the guy who is re-burning the chip, he has a day job as a computer geek contractor for the US Air Force.

 

[EMBPilot]

I just changed my bank 1 sensor 2, and i know ive read otherwise, but after the first tank of gas my milage when back up to a less pitiful 16-17MPG highway with my V8, it was down in the 15-14s. could have been driving conditions but i think it was the sensor repair.

http://www.explorerforum.com/forums/showthread.php?t=177483

 

[Lazzman]

Interesting, I might just have to change them anyhow.

It seems that there is still some doubt as to whether the rears can effect performance and gas milage.

Thanks

 

[aldive]

If it were me, I would turn off the rear O2 sensors.

All of my tunes have them off.

 

[lowflyingbird]

Hi,
Well I'm going to buy atleast a new set of upstream O2s. Aldive, I'm very interested in your mentioning of turning off the rear O2s, I dont quite understand how, or by who's programing. I'm currently building my new 5.0 to replace my 166kmi. engine. My old engine is well and good, just tired I feel. The new motor is being built with the late 80s Mustang forged pistons matching rotating assembly. The camshaft is going to be a 93-95 Cobra in conjunction with the 1.7 RR's. My cam choice is due to I'm also building an M112 Eaton blower package for the engine. The cobra cam is nothing hot on a light Cobra, but in a heavy Explorer, the 480 lift and 118 LS, should make for a good blower cam.I'm no beginner, done it before on other 5.0's. Getting back to the O2 issue, I will want to wait until I'm done playing with my new engine, but when ready who should I took towards for the tune you mention?
Please reply
Lowflyingbird

 

[aldive]

You will need either a chip or a computer tune to turn off the rear O2 sensors.

This can be done by a SCT dealer or by yourself if you have SCT;s Racer Pro package which includes Advantage III software for only one PCM code.

Good luck ....

 

[Lazzman]

I would prefer to have them off also. But if the rear 0'2s do nothing more than monitor the cats for efficiency why turn them off?

They don't seem to be a factor either way?

 

[Turdle]

Just something to throw in--

A friend of mine just recently had a high flow exhaust-cat system installed, along with new 02 sensors--

Either a cat, or a rear sensor is malfunctioning, indicating cat efficiency below threshold--and it seems that bank is also running a bit leaner---by almost 2 percent