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School me on downstream o2 sensors.


Dirtman

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I could make a long rambling and somewhat comedic story but Ill sum it up...

Im working on this hack job ranger my brother in law just bought. (Beautiful rust free southern truck just typical backyard mechanic repair jobs) 06 2.3 manual.

The cat was gone and just replaced with cheap flex pipe so first thing we did was go buy a complete new exhaust cat and to tailpipe. The upstream o2 was still connected but the downstream was unplugged and removed. I said screw the downstream for now and plugged the upstream back in.

My reasoning was that to my knowledge downstream o2 sensors don't effect the way the engine works, they simply monitor the catalytic converter. (I know some vehicles will stick the computer in open loop if the downstream o2 malfunctions, but they do not effect the fuel trims)

Anywhoozle the engine was still running crappy but lots of other factors going on however I said screw it and went and got a rear o2 since my brother in law wants the truck back to factory specs anyway. Hooked up the downstream and the truck came to life... ran like a champ.

The truck was going into closed loop mode prior to installing the downstream o2. So shat gives? Am i just wrong and downstream o2 sensors actually do effect the fuel trims?
 


sgtsandman

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They compare the gases before the cat and after the cat to set the fuel trim. That’s the simple and quick answer.

And as you found, they do make a big difference in a vehicle designed for there being an up and downstream sensor. The computer is hardwired for them.


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Dirtman

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But the downstream data would be effected by how well the catalysts is functioning. So as the cat slowly degrades so does your fuel trim even though there is no mechanical reason to be adjusting the fuel trim! The rest of the engine and components could be operating at 100% It makes no sense to me to try to adjust the fuel based on how well your cat works. Not argueing your statement, just not grasping the point or reason for the downstream to do that.
 
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sgtsandman

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That's why it will take someone more knowledgeable on the details behind it. I had to change the left cat on my 2011 because it $h!t the bed and was throwing a code that the aft O2 was bad. After some research on here an another Ranger forum, the general wisdom was that the cat was bad and not the sensor. I also had EXTREMELY fantastic fuel mileage right before the truck started throwing a code. All of a sudden I was getting 23.8 average mpg when the norm is 18-20 depending on the time of year and what tires and gear I'm running.
 

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In almost every official release of what it does the downstream is for nothing more than monitoring cat function.

However, a few years ago Ford had a recall on the 3.5 EB F-150s because a software problem was causing them to not get the full potential power and economy, and it was specifically related to the downstream sensors. They didn't explain beyond that, just said "flash the PCM".

I think it probably has something to do with a de-rate mode that is part of a catalyst preservation strategy that isn't widely advertised because it shouldn't be active absent some other issue that the customer would want fixed.
 

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In almost every official release of what it does the downstream is for nothing more than monitoring cat function.

However, a few years ago Ford had a recall on the 3.5 EB F-150s because a software problem was causing them to not get the full potential power and economy, and it was specifically related to the downstream sensors. They didn't explain beyond that, just said "flash the PCM".

I think it probably has something to do with a de-rate mode that is part of a catalyst preservation strategy that isn't widely advertised because it shouldn't be active absent some other issue that the customer would want fixed.
I can see crazy software like that on an ecoboost but on a 13 year old duratec? Im half tempted to go unplug the downstream on my 09 to see what happens...
 

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I can see crazy software like that on an ecoboost but on a 13 year old duratec? Im half tempted to go unplug the downstream on my 09 to see what happens...
Think about it though, the software running the two engine is pretty similar of it's core. Also these were 17s and 18s this recall was on, it was 13s and 14s. I've been on hiatus for over three years now.
 

Dirtman

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Ok eff it lets say weird software... The point of this was just the question, do downstream o2 sensors matter to fuel management. Your first sentence was

"Almost every official release of what it does the downstream is for nothing more than monitoring cat function."

That was my assumption for years... Ford, gm, dodge, whatever. Downsteam monitors the cat, not the engine.

Obviously the computer can get pissed off by the downstream but in functional condition, is it giving info to the ecm to change the fuel trims?
 

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Downstream O2s main function is as said to monitor the oxygen level coming out the the Cat converter after it has warmed up
Should run at .7-.8 volt, Low oxygen level

Whether or not this data is used in Long term fuel trims or short term is specific to the model of vehicle and it's computer software
So there is no correct "general" answer unless year model and software build are included

But the blanket statement of "its there to see if Cat is working" does apply across the board.

Upstream O2s are for instant feedback to the computer for open time for the injectors.
Their voltage should jump around 10 times a second, while the downstream remains steady, a fast changing downstream sensor is a bad O2

The computer does run engine Rich every few seconds to feed the Cat extra gasoline to keep it hot, the downstream O2 can be helpful for this timing to keep Cat at good operating temp.

Downstream O2 can also be used for Long term fuel trim, upstream O2 issues and misfire detection

If it is used with upstream O2s then it could cause rich running and loss of power if it started to act up
i.e. downstream O2 may cause computer to run engine richer than it should to keep Cat hotter than it should because downstream O2 is running leaner, .4 or .5, than it should

A misfire dumps extra oxygen in the exhaust manifold, that will show up at the downstream O2 as a sudden drop in voltage, which is harder to see at the faster changing upstream O2s

The Computer "sees" the change in crank or cam sensor speed with a misfire, the downstream drop confirms it was a misfire, so not a crank or cam sensor issue

Long term fuel trim(LTFT) is there so a vehicle can age gracefully :)
After engine warms up Computer calculates open time for injectors on the fly, and gets instant feedback from the upstream O2s
Say this is a newer vehicle
Calculated open time is 0 STFT
If O2 sensor shows Lean then STFT is change to +2%, injectors open 2% longer
If O2 showed Rich then STFT goes to -2%
Say the average for this drive of the vehicle was +5% STFT
So +5% is now LTFT

At the next start up cold engine would run memory fuel trim PLUS 5% longer open time
After warm up and O2 sensors are working, computer calculates 0 STFT injector open time and then adds 5% more and thats the new 0 STFT
So STFT should now vary like before, but average should be 0
LTFT has corrected for small vacuum leak, slightly lower fuel pressure, ???
Normal aging of the vehicles systems

Say it was a vacuum leak and when doing other repairs you saw the crack in the vacuum hose and fixed it
At the next round of driving the STFT would average maybe -4% with no air leak and at next startup LTFT might be +1% recorrected.

Downstream O2 running leaner than it should will cause LTFT to go up, its correcting for older Cat, giving it more fuel to keep it hotter.
Graceful aging :)
 

Dirtman

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Thanks Ron, nice easy to understand answer.
 

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