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Old 11-20-2008, 09:54 PM   #1
wrxsubiemod
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Default PO171 question?

Is it possible that this code would come on because of a leak at the header to up pipe gasket area. the o2 sensor is about 2-3 inches from this area and i wouldnt think that freash air would be traveling backwards through the exhaust stream! has anyone ever had this issue? I am going to change that gasket but i just want to know if it will fix my issue. I also did the stupid mistake of cutting and splicing a 04 sti 02 sensor to fit my 03 wrx! would this cause an issue. Im trying to diagnose my CEL. I just did a full sti swap, and i cant figure out where this code is comming from.
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Old 11-20-2008, 10:05 PM   #2
novacivic
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yes and yes. bot the leak and the splice will cause problems
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Old 11-20-2008, 10:07 PM   #3
UkNuck
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yes it can - but it's nothing to do with fresh air traveling through the exhaust

it's because the O2 sensor is an electrochemical cell that measures differences in oxygen concentration across it - if the external side is exposed to exhaust ( = low O2) it reads that the same as a lean condition ( = high O2) from the engine
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Old 11-20-2008, 10:48 PM   #4
wrxsubiemod
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Quote:
Originally Posted by UkNuck View Post
yes it can - but it's nothing to do with fresh air traveling through the exhaust

it's because the O2 sensor is an electrochemical cell that measures differences in oxygen concentration across it - if the external side is exposed to exhaust ( = low O2) it reads that the same as a lean condition ( = high O2) from the engine
Not following you? So a leak that is just above the O2 will give me a cell. is that what everyone agrees with? Im really just trying to figure this out. I could understand that maybe the O2 sensor is different, and i should change it out. but why is it that this check engine light will come on only at freeway cruising! Sounds like a leak. and i am sure that their is one at the header to up pipe gasket.
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Old 11-21-2008, 08:30 AM   #5
UkNuck
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Well the technical details don't matter - the important things are

1) you should use the correct OEM 22641AA042 or EXACT EQUIVALENT Denso part 234-9011 - I don't know if the STI one is the same or different, but the fact that you cut the harness suggests it's not the correct part

2) you should fix the leak

The reason it only throws a CEL at cruise is likely because that's when the ECU is using the front O2 reading to maintain closed loop AFR.
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Old 11-25-2008, 10:31 PM   #6
wrxsubiemod
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UPDATE. ok it wasnt the exhaust leak. and it wasnt a intake leak i found ither. it also wasnt the mass airflow, or the IAC. I cleaned my injectors, wasnt that ither. I took it to get tuned twice. didnt fix it. the Tuner told me my O2 sensor was reading fine. Nope he was wrong. i just swapped it with my buddies and no more check engine lights. woooow!
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