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Discussion Starter · #1 ·
Hey Guys, I've been working on this for a few weeks now and about to pull my hair out. This symptom began with the installation of my 3" exhaust. Here's what happens.

With the rear O2 sensor plugged straight into the bung, I get Bank 1: 02 (Lambda) correction behind catalyst: Control Limit Reached. P1176 - 001 - Upper Limit Exceeded

Car runs generally okay but I get kind of a loping idle on a 4-5 second interval and it's moderately hard to start when warm. Takes 5-8 seconds of cranking with throttle depressed about halfway before it'll finally fire up. Starts fine when cold.

Next I started playing with spark plug non-foulers, after reading other posts and figuring if I'm flooding out due to these 2nd O2 sensor errors then the logical way to fix it must be to space the O2 farther away from the exhaust flow like everyone else has. I have now played around with distancing the O2 sensor anywhere from about .5" to 1.75" farther away from the bung with various combinations of welded on nuts, screwed in non-foulers, etc. With ANY distance of spacing I get the following VAG-COM and symptom:

Catalyst System; Bank 1: Efficiency below threshold P0420 - 002 - Lower Limit Exceeded

The car becomes VERY hard to start warm when left for anywhere from 3 minutes on up to 25-30 minutes. Almost every time I have to unplug the injectors, crank it for 20-30 seconds to clear the cylinders, plug the injectors back in, and then it'll start again using the throttle-half-depressed method.

Using both setups (O2 through non-foulers or screwed straight into the bung) the car has died without warning a couple times while idling.

My complete setup is this:
K04
GIAC software
2 1/2" FMIC
3" MAF
380cc injectors
tt BOV
custom 3" turbo-back exhaust w/flowmaster cat

Has anybody else fought with this before? Does the rear O2 sensor really control enough of the ECU function that it can tell the car to flood out every time it tries to hot start?


Any help is appreciated. Thanks to all of you for your help over the years. Been a member since 2006, just no need to post very much since there's so much information here already once you figure out how to search.
 

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Did you put the O2 sensors back in the right spots? The second O2 sensor has no connection to fuel, it is there to see if the cat is working. The first O2 sensor does all the air/fuel stuff. Cat efficiency codes will not make the car run poorly. Do a boost leak test and check for exhaust leaks with seafoam and a rag on the exhaust tip.
 

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Discussion Starter · #3 ·
Hmm, Thanks for the suggestions Weehe. Sounds like you're asking if maybe I got the front/rear O2s switched, as in the front is screwed in behind the cat and vice versa? That's a good question. I'll check it out in just a bit. That'd be a super easy fix if so! Did a boost test once several weeks ago trying to figure it out too but never hurts to try again. Thanks again for the suggestions. I'll post my results later today.
 

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Discussion Starter · #4 · (Edited)
oxygen sensors plugged in securely in their appropriate locations, no exhaust leaks, no boost leaks. found a vacuum leak under the intake manifold at that little tee but replaced it, clamped it and still have the problem. VAG-COM block 001 seems to measure the O2 sensor +/-10% and what I'm seeing is that it bounces around in the negatives from -4%--28% or so until suddenly it goes to 0.0% for the entire duration of the rest of the run. I am sort of leaning towards a bad front O2 at this point but that doesn't make complete sense to me because it's not throwing any codes. the only codes I get are rear efficiency/limit reached. I don't have a fuel pressure gauge but the FPR seems a little unlikely anyway since it was working fine until I went K04. anything else I'm missing that might affect fueling?
 
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