O2 Sensors: A parable.

There was once a blonde who had uncanny mechanical skills.
Anything that broke, she somehow knew how to fix it.. but when she bought a car for the first time, it taught her a lesson.
She noted that the gauges moved at various times, the longer she drove the car the higher the temp gauge went till it stopped in the middle, but every time she let the car sit for a time, it would start at the bottom again.
But the one that was labeled "fuel" didnt act like that. It started at F but it kept reading lower and lower...until one day, it went to E and seemed to stay there...
So she went to her car dealer and bought a new gauge, ...
- Changing an O2 sensor for a rich or lean condition is just like buying a new gauge for a low tank of gas.
And any mechanic or autostore clerk that doesnt tell you that is a crook
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- Yes, I'm a crusty old geezer curmudgeon.. deal with it! -

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not necessarly true, quite often the (GM anyway) sensor will start to fail and give a lean feel and code.(sensor tells comp it is rich) every time replaceing the sensor cures it. KB
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Nothing to do with GM. They all do it. The computer will see a lean condition, reported by a bad O2 sensor, and start making the mixture richer. After a short time the mixture will be full rich and the computer will report its not able to make it rich enough.
Al
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Big Al opined in

Well, Am I red faced...Too bad you cant delete "posts" on groups!
I've only heard of "no switching" and never had one fail that way or seen it on here.
Learn something new alla time!
;)
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You're not wrong Backyard, sensors can fail in the manner these guys are talking about, but throwing in a sensor every time you get an O2 code will result in a lot of good O2 sensors being replaced. All sorts of things can trip an O2 code. Bob
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Not so fast... Am I correct in assuming that you authored this thread based upon the post by someone who shot-gunned three O2 sensors based upon trouble codes PO171 and PO174 being stored? The odds of two pre-cat O2 sensors failing exactly the same way and falsely reporting identical mixture conditions at the same time are extremely remote. On the other hand, we've been posting here for years such causes as dirty MAF sensors and rotted PCV hoses, conditions that -do- trigger simultaneous PO171 and PO174 codes.

O2 sensors 'skew,' that is a known fact. OBD2 emissions systems are advanced enough that such skewing is caught and reported by the numerous O2 failure codes indigenous to OBD2.

I worked on a 97 Tahoe earlier this week, 120K miles, it had failed our state IM test which for OBD2 vehicles consists of plugging in and interrogating the PCM for faults. Initial stored codes were for Bank 2 Sensor 2 heater circuit (I don't recall the exact code) and a PO430 Bank 2 catalyst efficiency code. PO430 shouldn't be able to set if there are any O2 sensor related codes so, something is fishy to begin with. I did some routine testing to verify whether the B2S2 heater was working (it was), I cleared the codes and drove the truck thru my patented OBD2 drive cycle test route to re-set all the readiness monitors. New codes set were PO171 and PO174, lean mixture on both banks. Scan tool showed long term fuel trims to be maxed out at 150 counts, when read as a percentage amounts to +25%. On GM trucks of this vintage, the fuel trims can be re-set with the scanner with the engine running, after re-setting the fuel trims, they would immediately begin to increase to their maximum value. The O2 sensors could be driven high by adding propane into the PCV breather and checking Mode$06 on the scanner showed no anomalies WRT O2 sensor functionality so there is no reason to believe that they have skewed. Obviously, this truck was starving for fuel.... Checked fuel pressure, it's in spec but the engine stumbled when I connected my FP gauge (it's got a quick connect).
It's interesting to note at this point that the vehicle owner had already visited the local Chevy dealership where she was advised that the truck needed new catalytic convertors and 4 new oxygen sensors $$$$$$$...
I replaced the original from the factory fuel filter, cleared the trouble codes and re drove my patented OBD2 drive cycle test route, truck runs great, fuel trim numbers are within 6% of ideal, towards the end of the drive cycle I see that all but readiness monitors are showing "complete." In to the emissions test station I go. BINGO, passed with flying colors!
Oh yeah, the B2S2 heater code.... The transmission was rebuilt a few weeks ago at the same Chevy dealership, I found the wiring harness incorrectly routed near the transfer case, my suspicion is that at some point, the truck was started and run with the back half of the B2S2 harness disconnected, setting the false B2S2 heater code (best guess).
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Thank you.... yes that was my point!!!! And you, too Bob.
I'm obviously not a pro auto tech - thus the handle "B.M"- but I WAS both a logic and systems engineer/analyst for over thirty years, lucky enough to be interested in what I did and that's why I was a success in staying current in my field.
For hobby, (Ford turbo EFIcars) besides doing everything, but AT rebuild, on my own driver and project cars.
A lot of my knowledge IS gathered on here and in ink, and no matter my "for effect" MEA CULPA, I stand by my original statement. Note the "winky" at the end.
The original poster got taken... If I was a parts counter guy, I couldnt have sold the guy three O2 sensors without asking him if they were REALLY needed.
And it DID just increase the info available on this forum, didnt it!
cheers!
aarcuda69062 opined in wrote:

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Good point BM. But.....If that sensor has alot of miles on it, just replace it anyway......They dont last forever and they get "lazy" over time. Even if they dont 'even' set a code :)
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Scott M opined in

Yes,and fuel gauges sometimes fail, too... but I'd check my fuel level before I replaced it.
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O2 sensors get repaced almost every week at the shop I work at, but we get O2 codes about every day. Now O2s get lazy as they wear out but, we find more often other things like exhaust manifold leaks, vacuum leaks, EGR leaks, fuel presure regulators leaking, ect. that actually cause the mixture problems and the O2 read it correctly. O2 heaters go bad, they switch slow, stick lean (low volt), continually and intermitant. Most of us have more time than money and the only thing lost would be time to do a little troubleshooting, which most of you sound like you could do or ask here. Heck it sounds like some of us actually have fun troubleshooting.
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