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
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
- Yes, I'm a crusty old geezer curmudgeon.. deal with it! -
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.
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.
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
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
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).
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
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!
aarcuda69062 opined in
- Yes, I'm a crusty old geezer curmudgeon.. deal with it! -
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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