I'm confused about how oxygen sensors work

[OldEuroCarLover]

Over the years I've replaced many o2 sensors in many cars. Often I'll replace the upstream sensors around the 100k-150k mile range as a maintenance item. The upstream replacement--typically done around the time of other items such as trans flush, ECT replacement, filters, spark plugs, etc--often results in slightly higher fuel mileage. (Stating all of that because it obviously doesn't directly implicate the replacement of the o2 sensors as the cause of improvement.)

Which leads to my question: do the downstream sensors ever matter for fuel injection calculation, or are they simply only there to indicate a bad catalyst? This has always been my understanding, and I've gotten up to around 250k miles without replacing downstream sensors.

 

[bdcardinal]

The rear O2s are there only to monitor catalyst efficiency. For a while Ford would refer to them as Cat Monitors.

Edit, to expand I have read that some applications will take input from the cat monitors to try and change fuel trim to see if the cat is bad, but this is not common.

 

[JohnG]

The catalytic converter needs oxygen to work correctly. There is an incineration process going on in the CAT. The O2 sensor ensures that there is sufficient oxygen left over from the combustion process, whether by injection, or some other route, to support the needs of the CAT.
I can't see how it might impact fuel economy in any way though. It's not like a MAF sensor or anything that controls air or mixture PRIOR to combustion.

 

[CharBaby]

The way that I understand it is that they just relay information to the computer.
The computer determines what is in spec or not. As sensors age due to too many heat cycles, they get "lazy" so to speak and the computer gives false readings...or doesn't know what to do. So it triggers a CEL and it's up to you or the tech to find out what is going on...if I said that right.

 

[thastinger]

Originally Posted by JohnG
The catalytic converter needs oxygen to work correctly. There is an incineration process going on in the CAT. The O2 sensor ensures that there is sufficient oxygen left over from the combustion process, whether by injection, or some other route, to support the needs of the CAT.
I can't see how it might impact fuel economy in any way though. It's not like a MAF sensor or anything that controls air or mixture PRIOR to combustion.


O2 sensors control your short term fuel trim tables and data from them build your long term fuel tables.

 

[supton]

Originally Posted by bdcardinal
The rear O2s are there only to monitor catalyst efficiency. For a while Ford would refer to them as Cat Monitors.

Edit, to expand I have read that some applications will take input from the cat monitors to try and change fuel trim to see if the cat is bad, but this is not common.

That's kinda the problem here: what one OEM does might only apply to them. Some things are common enough to be a rule, others not so much. Have to hold rules of thumb with some level of skepticism. No sooner do you learn the rules that you learn of some exception...

Some time ago I got bored and hooked up Torq to look at the O2 sensors in my Camry. Rear O2 was dead. Not slow moving, I mean, dead--it was sitting at zero volts. I went on line, ordered the sensor. Replaced it a week later. Now I have no idea when it died, but it had a couple hundred miles and a bunch of restarts before I replaced it. And... it never set a CEL.

How's that work? (other than to my advantage!)

 

[The_Nuke]

The rear O2 sensors in Dodge's LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the "used by PCM to adjust fuel trims" argument IDK, but it's definitely different than what I'm used to.

 

[CT8]

Basically O2 sensors work by generating a voltage between the atmospheric o2 and the exhaust gas 02.so the computer can adjust. The post converter sensor is a teltale to let the computer the pre cat sensor is functioning.

 

[OldEuroCarLover]

Originally Posted by bdcardinal
The rear O2s are there only to monitor catalyst efficiency. For a while Ford would refer to them as Cat Monitors.

Edit, to expand I have read that some applications will take input from the cat monitors to try and change fuel trim to see if the cat is bad, but this is not common.


This is what I've seen in a few places, and has me confused.

I've always looked at the downstream sensors as just that: cat monitors. A code means the sensor or the cat is bad, and never anything else. But it's starting to seem like it's not always that simple.

 

[michaelluscher]

Originally Posted by The_Nuke
The rear O2 sensors in Dodge's LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the "used by PCM to adjust fuel trims" argument IDK, but it's definitely different than what I'm used to.



Chrysler is a weird exception

Some of them do use the rear post cat O2 to influence the front sensors decision making on fuel trims

 

[OldEuroCarLover]

Originally Posted by michaelluscher
Originally Posted by The_Nuke
The rear O2 sensors in Dodge's LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the "used by PCM to adjust fuel trims" argument IDK, but it's definitely different than what I'm used to.



Chrysler is a weird exception

Some of them do use the rear post cat O2 to influence the front sensors decision making on fuel trims


Is this strictly Chrysler? When did they start doing it? Do Mercedes or Fiat have a similar system?

 

[shanneba]

I've seen reference to the rear O2 sensors adjusting fuel trims on some BMW models.

Rear O2 Sensor Fuel Trim
The rear oxygen sensor, located after the catalyst, is used for fuel trim corrections on some
OBD-II vehicles. By virtue of its location, the rear sensor is generally protected from high
temperatures and much of the contamination that affects the front oxygen sensors. In addition,
the rear sensor sees exhaust gases that are equilibrated - they have already been converted
by the catalyst so that there is very little residual oxygen. This allows the rear sensor to
respond to much smaller changes in exhaust gas oxygen content. In turn, it then possible for
the rear sensor voltage to remain near the 0.45 volt switchpoint. This characteristic allows the
rear sensor to be used for fuel control. Under steady rpm and load conditions, the short term
fuel trim bias can be adjusted so that the rear sensor voltage is maintained near the 0.45 volt
switchpoint.

Here is a link to download the Baum Tools BMW 39 document:
BMW E39 Fuel Trims

 

[eljefino]

Originally Posted by The_Nuke
The rear O2 sensors in Dodge's LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the "used by PCM to adjust fuel trims" argument IDK, but it's definitely different than what I'm used to.


You probably have a 3-way catalyist with the rear part doing NOx. Is there an air injection port system?

O2 sensors fail by getting "lazy", slower to switch. A correctly functioning catalytic converter is confirmed by a lazy rear o2 sensor reading, so replacing it with a "sharp" one can bring you peril in an older car.

 

[bdcardinal]

Originally Posted by The_Nuke
The rear O2 sensors in Dodge's LX vehicles is mounted dead center in the cat, which is different from every other vehicle I ever had where they were actually mounted downstream of the cat. How that factors into the "used by PCM to adjust fuel trims" argument IDK, but it's definitely different than what I'm used to.


I have seen some Ford and Mazda cats like that as well.

And not to add to the confusion, but I have seen some Mazdas where there are 3 sensors per bank. An A/F sensor pre-cat, a sensor in the cat, and one post cat.

 

[Hall]

Originally Posted by OldEuroCarLover
Over the years I've replaced many o2 sensors in many cars. Often I'll replace the upstream sensors around the 100k-150k mile range as a maintenance item.

Truly curious, why replace them if they're still working ? It's not like when one fails it will leave you stranded.

 

[spasm3]

Originally Posted by OldEuroCarLover
Over the years I've replaced many o2 sensors in many cars. Often I'll replace the upstream sensors around the 100k-150k mile range as a maintenance item.



I do the same , around 80-100k I replace the upstream sensors.

 

[Propflux01]

Do you notice any difference when doing this?

 

[spasm3]

Originally Posted by Propflux01
Do you notice any difference when doing this?


Usually a 1-2mpg increase.

 

[OldEuroCarLover]

Same here. Like I said in the first post, I've generally done this along with multiple other maintenance items. But a small increase in MPGs is common.

I do the oxygen sensors preventatively because it's easier to plan the maintenance and do everything at once than it is to respond to a trouble code at a random time. They will go bad at some point, and 100k to 150k is generally when that happens.

For some stupid reason I've never preventatively replaced the fuel pump on a car, and it's bitten me in the [censored] twice.

 

[Falkirk88]

Over time the oxygen sensors slow down, which leads to less engine efficiency. To test an O2 sensor, you would use a scope. Stab the throttle and measure the time the O2 goes from rich to lean. This will be measured in milliseconds. If the sensor is within range, it is working fine and should be running fairly efficiently.

When the millisecond reading is outside of the spec. it is not giving the ECM the best and fastest information to stay the most efficient. Depending on demand, the ECM tries to keep the combustion at stoichiometric, which is 14.7 to 1. Replacing lazy oxygen sensors will allow the ECM to get faster information, which then allows the ECM to control the fuel management that much better. This will equate to better fuel mileage.

As far as the post CAT O2's, as has been stated they only monitor the catalyst efficiency. What typically sets the CEL is when the post cat O2 mirrors the Pre cat O2 in readings. This is a sure fire way to show the cat is dead.