failed emissions test

I recently had the check engine light illuminate and the test displayed codes p0171 and p0174. I changed all 3 o2 sensors and the light went out. I took the car for the vehicle emissions test and it failed with the same
codes displayed but no check engine light. Later that day the check engine light illunimated again. Any idea what I should try now. A co-worker told me that sometimes it takes about 70 miles for the code to completely clear from the computer. Any imput?
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In article

Both are lean mixture codes, they won't necessarily be fixed by replacing the O2 sensor(s). You'd be better off fixing the lean running condition that set the codes to begin with.
The codes need to be manually cleared either with a scan tool or by disconnecting the battery, otherwise, it can take up to 50 key cycles for them to erase themselves. If the codes are cleared by scan tool or battery disconnect, you'll have to drive the vehicle for a few days to re-set the diagnostic monitors before returning for your emissions test.
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aarcuda69062 opined in wrote:

Good info there, aarcuda!
Milpay, what you did essentially was shoot the messenger's horses. What AARCUDA MIGHT have said is they almost NEVER are fixed by replacing the O2 sensors
You should have serious words with whoever advised you to buy and replace those sensors
Any other symptoms?
Usually the car isnt running quite up to par when that's set off.
--
- Yes, I'm a crusty old geezer curmudgeon.. deal with it! -

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Yes there where other symptoms. The Idle speed whould fluxuate 700-1000 rpm and while driving you could observe the tach fluxuating as well as feel the car surge. Since replacing the o2 sensors the rpm is much steadier now with occassional 100-200 rpm.
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Thanks for the reply. Yes there where other symptoms associated with the check engine light. The car would not maintain a smooth idle speed. The idle would fluxuate as much as 700-1000 rpm at idle and while you where driving you could feel the pulsing of the car as well as see the surging on the tach. Since changing the o2 sensors the Idle rpm is much steadier now at about 600 rpm with occassional fluxuations of 100-200 rpms and no surging while driving.
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From what I understand the lean condition is probably either caused by not enough fuel (perhaps a clogged fuel filter) or too much air (as in a vacuum leak) do you have oil leaking out of the valve covers or anywhere else? If so, the vacuum leak may be in the same place.
Just some ideas to look into.
-JCM900

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