O2 Sensor Failure: Front or Rear?
 

I am partly creating this thread to see the list of 'related threads' generated for a match, but might leave it up here.

I've known our search engine is often less than helpful, but try searching for "O2 sensor". You get 6,040 threads. Every time the word "sensor" is mentioned :wallbash:.

Anyway,

I believe my rear sensor is failing/failed, but trying to confirm that I am accurate, and it isn't the front sensor.

No persistent CEL to pull the code, but my reasoning:

1. Getting misfires back in December prior to my complete CAT failure
2. Replaced Coils, plugs, wires, filter, cleaned MAF
3. still issues, MPG drops to ~9mpg
4. Determine cat failure
5. remove cat install loaner midpipe (friend's pipe had his O2 sensor on it)
6. back up to 22-24mpg, full power (plus some), no misfires
7. dealer replaces cat end of January, using my old O2 sensor
8. I get misfires again, mileage drops to ~17-19
9. misfires continue, still puzzled like crazy, mileage keeps dropping and I am clearly running way too rich
10. someone suggests an O2 sensor failure, as I wasn't aware that they were read/write interactive, rather than just read only for emissions
11. If a sensor would have gotten damaged from the clogged cat failure, the rear sensor makes sense

Any takers?

So much for related threads....sigh.

The rear sensor has no impact on performance ie mileage, misfires, etc plus it will throw a CEL if bad, all it does is monitor cat discharge gases. You can pull it and run without it even, no impact.

you can't guess a solution, you need someone with the skills and equipment to properly diagnose the vehicle

wrt misfires, have you tried restting the e-shaft profile?

It almost sounds like a clogged cat or exhaust to me, but again it takes a qualified diagnosis to figure these things, don't go swapping parts etc just because the forum yingyangs start spouting off guesses

Hmm.... so when you took the cat off and put the midpipe on, everything was right in the world.

How did you determine that its the 02 sensor when you swap the cat for a midpipe and its fixed? I assume you'd be using the same 02 sensors. . .

He replaced his cat already from the dealer. First cat was switched out with a loaner which resulted in the car running normal again. Once the dealer put in a new cat after he was done with the loaner the problem was back again even though it was brand new

I would think that isolates it to just around that area causing the issue. Other than the O2 sensor what else is left Team that could cause an issue there? E-shaft is the only other possible culprit or is there more?


RIWWP: Did you get the CEL code looked up yet to narrow down what it reports?

There you go

Did the problem start back immediately after the dealer repair or sometime later? Are you getting misfire CELs? What are the conditions that the misfires occur during?

ie ask questions, eliminate assumptions



.

My cat has ~1,500 miles on it (1/29?/2010). Literally brand new. My coils, plugs, wires, air filter have ~3,500 miles on them (12/24/2009). I have reset the e-shaft sensor and fuel trims several times throughout this whole process, probably about once every 2 weeks.

I'm not a fan of guesses. Hence this thread. Anyone in the B@W thread can tell you how analytical I am :)

The member that loaned the midpipe has his 8 in storage, and was visiting relatives for christmas, so he gave me the code and I pulled the midpipe off his 8, leaving his O2 sensor on the midpipe. I pulled my cat off and left my O2 sensor on my burnt cat. Installed his midpipe with his O2 sensor. I didn't have a solid missing-cat cel for most of the time I had his midpipe. Not a single flashing cell popped up. If nothing else, back up to 22-24mpg tells me that everything was perfectly fine.

When the dealer replaced my cat, I imagine that they used my old O2 sensor, since it wasn't noted in the invoice ($0 final, under warranty).

I got the first misfire on the way home from the dealer the day of cat replacement.


No solid CEL to pull it? Unless you can read a flashing cel after one popped? I have a scanalyzer on order that will grab the code I imagine if I don't get this fixed before it gets here.

Agreed :)

First misfire was on the way back from the dealer after cat replacement. The misfire is always in high RPM, generally >8k, always under full WOT load or immediately as I lift prior to shifting. My mileage has also dropped to ~14-15mpg, which I believe to indicate that something isn't healthy.

I analyze the hell out of everything, and didn't think the O2 sensor was anything other than an emissions check, so wasn't considering it until last week. The only thing left otherwise that hasn't been replaced is the engine :)

"The purpose of the front oxygen sensor, which is mounted before the catalytic converter, is to measure how rich or lean the gases are as the gases exit the combustion chambers. Depending upon whether the exhaust gas is lean (high in oxygen content) or rich (low in oxygen content), the amount of fuel entering the engine is adjusted by the engine management computer to try and maintain an ideal mixture that produces the lowest emissions output from the catalytic converter."

Quoted from one source. Sounds like rear is just a monitor whereas the front one is going to be the problem out of the 2 of them if it's bad. Since it should be in front of the converter then it was untouched during the swap and didn't influence the problem depending on what got removed. I haven't looked under the car in this area yet to see where precisely it is in the exhaust.

^ Part no. 9

The front o2 is in the exhaust manifold, before the cat/mid pipe flange.

I guess it would be your front o2 sensor that needs replaced first. . .

So... if your front sensor is bad its causing your car to run rich, which is caused your first cat to fail. . . then with a midpipe, even though your were still running rich, it didn't effect you as much because your engine wasn't working to push all those exhaust gases through your cat.

Then, cat gets put back on, you're still running rich, but your cat isn't clogged, (yet) so your mpg drops back down. . .

Maybe???

I don't know, if it were me, I'd be shopping for a midpipe, but we also don't emissions testing in backwoods Ohio.

Ok then, have it your way ...

We're just discussing the location and functionality of the 2 O2 sensors Team, we're not saying they are the culprit.

I asked in a previous response from your experience what else could be the problem when removing the cat and installing a mid pipe to eliminate it and the problem is gone and when reinstalling a cat, it's back again.

A midpipe is in my future at some point, but if I have a bad sensor, getting a midpipe won't fix that.

I am really just looking for a way of confirming which one is bad (without having to go through this and lots of out of pocket cash) with the dealer. I can replace both as well, it's an option, but as always, I am more interested in the WHY than I am in the solution.

The discussion, and challenges, is what I encourage here :)

Give your input openly Team, I value it. I refuse to take a single line of information and assume it's fact from anyone (including you :) ).

Heading out of the office for a couple hours, so won't be able to contribute for a bit.

This tells me that it is the front sensor, not the rear, that I have an issue with.

Anyone else have any feedback on that?

guess from a yingyang : front o2 sensor

I had one fail recently and this is what happened :

AFRs went all over the place in open loop , mostly lean in my case (per my wideband guage)
When I logged the o2 sensor (front) via scanalyser I found that it functioned normally for a while but when it got hot it would stop working and give a 14.7 flat line reading no matter what .

When you fitted your mates midpipe you didn't happen to disconnect the battery did you ?

FWIW I ran my (turbo) car with no o2 sensors connected at all for about a month while i waited for a new one . It ran fine .

I would agree... but like Team said... ask questions to eliminate assumptions.

Most times I would say a misfire is coils and plugs... but since those are new... I would try a new front o2 sensor...

This makes sense. My 8 runs "perfectly fine" during light throttle warmup in the morning, and for about a day after resetting the e-shaft profile and fuel trims (either pedal pump or battery disconnect), well, idle is rough as expected, but the rest under load is back to normal for me.

Looks like my original assumption about the rear sensor was incorrect, and it is the front sensor. (I might just order both). I guess when I had the midpipe, the heat buildup there was lower than with the cat (both new and clogged).

And yes, I did reset everything on my 8 installing the midpipe loaner.

See, this is why I made the thread. Challenge my assumptions! :)

you can run without either one, in the case of the front one it will default to open loop

in either case you should get a CEL if it doesn't operate as intended, even if the fault is internal

the only reason I said "have it your way" is because I've watched too many people swap out parts based on conjecture and just end up with a big hole in their wallet and the problem still not resolved i.e. "I might just order both" :rolleyes:

fwiw, it doesn't make sense. rather it's just what you wanted to hear



.

So what else might this be then? The only thing not addressed is the engine itself, which I agree is certainly a possibility. I can easily end up with a big hole in my wallet from taking it to a dealer for them to diagnose, potentially without any resolution at all for my money. The dealers in the area are general morons. Even that cat replacement I had to drive an hour and a half to a dealer that was willing to take my word for the problem.

I could burn money on conjecture and keep replacing ignition pieces, that are already new, but I don't see the wisdom in that at all.


I've got to spend money somehow to get this resolved, and until I do, the extra gas usage is spending it for me anyway :). If it isn't the ignition or O2 sensor, then the only thing left I am aware of that could be causing this is the engine itself, and due to other indications, I have trouble believing that my engine is failing.


Perhaps I am just not reading inflection and tone very well through written word, but if you have an idea about what this might be, but need further information to help pinpoint, ask away! I value your input quite a bit.

I have a hard time believing you can run without the front O2 sensor, especially when it affects the fuel trim and if it's bad or you remove it will result in making the ECU guess the correct ratio.

Searching on this forum shows people who've had idling/mpg related issues until they replaced the front O2 sensor and the problem was resolved. But I'd at least wait until the scanner comes in to see what you can pick up RIWWP.

It will run without the sensor - I have done it . All the sensor does is modify the fueling . What happens is the ECU gets stuck on whatever fuel trim it last made with the functioning sensor . Or if you reset the trims you get whatever the stock tune delivers without any trim .

Great read.
I started out with a P0335 - Eccentric Shaft sensor problem a day before I had to go for my emission check.
AFR's all over the place.
Got things settled down and during the emission test car threw a P2270: Rear HOS2 stuck lean.
Car failed emission test.
Tech said likely the cat is toast, I tend to agree the exhaust is stinky. (RP High Flow cat)
Anyways I am in the process of putting the OEM cat back on but it had been suggested that my rear O2 sensor may need replacing as well.

So as an effort in enlightenment ... thanks everyone to the info in this thread.

I do have a question: Is there a way to visual inspect or using an ohm meter to test the O2 sensor?

My thread where all my troubles began:
https://www.rx8club.com/series-i-trouble-shooting-95/p0335-eccentric-shaft-position-sensor-circuit-problem-car-will-not-idle-194505/

I never ended up with a different answer on this.

My issue ended up being my compression was just garbage, found out yesterday. See the thread here:
https://www.rx8club.com/series-i-tech-garage-22/failing-compression-isnt-only-test-your-engine-needs-fail-194710/

^^^
Dude that sucks.
Low compression has been suggested to me as well... and I have a 2005 MT 6SP and its Whitewater pearl!!!

Are the front and rear o2 sensors the same part number? I asked a guy at a autoparts store and he told me the front is called a air fuel ratio sensor and the rear is a o2 sensor. Is this true?