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Old 09-01-2008, 07:42 PM   #1
Blackwidow330CI
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O2 sensor gone bad without codes??????

MAYBE someone can give me an answer with there experiences? I tried Bimmerforums.....no luck. I unplugged my O2 sensor on bank 1. My question is can an O2 sensor go bad without giving a code? This is what was happening.Check engine light comes on. 8 codes,O2 sensor bank 2,all kinds of misfires,fuel cut off's and sluggish,plus hesitation, and lack of power. This what I did. New air filter,spark plugs(good ones OEM) Boot to TB changed. Cleaned ICV. Changed DISA(air flap in intake) it was busted! Pulled off intake....Interchanged injectors and coils(checked resistance on coils...all good) NEW AFM$$$$ NEW cam position sensor....intake side. Changed the oil separator valve and hoses. Changed all vacuum hoses. Did the fuel filter last week.....that helped alot! changed O2 on bank 2. Checked the wiring harness to the coils....all good. So this morning I unplug the O2 bank 1. Whamo...she runs like a champ! No misfires, no lean or rich codes, no hesitation. Anybody have this kind of thing B4???? Bad part= no codes??? Idle is a bit low....BUT I already ordered the VANOS seals. Thanks in advance....
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Old 09-01-2008, 07:58 PM   #2
ari00
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Yes!! Although you've already figured this out, this was, in fact, my case last year. It took a while...about a couple months for a code to come up via the SES light. I know many forum members here have replaced one or another part of the ones you mentioned and it cured their rough idle, etc.

But for anyone who is 90k miles or more and plans on keeping their car for a long while more, especially for anyone having any of the symptoms above, do yourselves a favor and change your 02 sensors! You'll have to do it sooner or later.
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Old 09-01-2008, 10:21 PM   #3
Blackwidow330CI
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Thanks for the quick reply. When I bought the car it gave me bank 2 O2 needed to be changed. So I did. I realize now that I should have changed both. It is just funny how......you would think the DME would pick it up?!....Hopefully though when I do the VANOS seals........she should idle alot better.....so people say. BTW....I have 212,000km on her....or around 120,000 miles. I will change it tomorrow.....Thanks again....
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Old 09-01-2008, 10:33 PM   #4
ari00
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Quote:
Originally Posted by Blackwidow330CI View Post
Thanks for the quick reply. When I bought the car it gave me bank 2 O2 needed to be changed. So I did. I realize now that I should have changed both. It is just funny how......you would think the DME would pick it up?!....Hopefully though when I do the VANOS seals........she should idle alot better.....so people say. BTW....I have 212,000km on her....or around 120,000 miles. I will change it tomorrow.....Thanks again....

No worries, glad to be of help. I changed all 4 of mine at the same time to avoid this issue. And yes, I believe after hearing everyone here who has replaced the VANOS seals that you should have a smoother running car. Mine still runs like new at 110k miles, but that could change soon. I'm going to look into the seal replacement solution offered here pretty soon. Good Luck with yours!

Ari
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Old 09-02-2008, 07:59 AM   #5
Blackwidow330CI
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Another theory. If my precat o2 was going bad or is bad.......MAYBE the POST cat O2 sensor picked up the mixture and the DME read that......That is why I did not get codes. I did some research and supposedly the post or after cat. O2's only control emmisions? My 2 cents...Shall repost when I change it.
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Old 09-02-2008, 10:44 AM   #6
hummer
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I believe the pre cat O2 sensors are the ones the DME uses to control the mixture and emissions. The post cat O2 sensors monitor the cat function.
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Old 09-02-2008, 11:27 AM   #7
tony325ci_51
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Quote:
Originally Posted by Blackwidow330CI View Post
My question is can an O2 sensor go bad without giving a code?
Yes, a code will be given if an O2 sensor fails, that is, cannot be seen or is giving no data to the DME.

If the sensor is going bad and giving slightly false or incorrect data, how is the DME to know that data is incorrect, hence no code.

There are ways of telling if a sensor is going bad with the right diagnostic equipment I believe.
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Old 09-02-2008, 11:45 AM   #8
Blackwidow330CI
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Thanks guys....Good points there. Even if the O2 works .....BUT has not failed the DME can still receive data.....but incorrect data if it is on the fritz. changing it today.....Will update l8tr
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Old 12-09-2012, 10:41 AM   #9
msdg137
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Digging Up an old thread so that others may help diagnose similar problems. I have had this very situation with my car and felt I should cover it in more detail. I purchased the car with an incomplete motor. Repaced it with one with 64k. Once installed the motor would run well on initial start up(open loop) and then stumble and misfire when it began running off the o2`s(closed loop).
01 330xi
Only codes were random misfires.
Replaced all coilpacks and sparkplugs with spares and got misfires on #2 and #3.
Shut off 2/3 injector on the scanner and no change in Idle. #2 and #3 were doing nothing.
Checked fuel pressure and it was holding 50psi with no debris in the rail.
Watched the up stream and Down Stream o2's on both banks simultaneously and had little to no response when all should be fluctuating.
Checked for wiring mishaps back to the DME and all was well.
Unplugged the O2 sensors keeping the car in open loop runs and Idles fine.

Conclusion: The front bank of o2 sensors were reading so poorly that the injectors were not delivering fuel to the motor. They did not throw codes however because they were connected and the heaters were functioning properly.

I will follow up with my new o2 sensor readings.
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