# Downpipe = Check Engine Light?



## 25psi (May 7, 2003)

Hey guys.
Well, on Saturday I had Megan racing 2.5" DP installed in additionto my 2.5" cat-back by same company. So, now i got 2.5" exhaust system all the way from turbo back. Sounds good so far right? I expected so as well.
Right away i noticed that something is "Different" (I am strugling to find better word to describe). I could hear turbo whine and exhaust was louder - Those are Ok and do not bother me. But something else felt a tad weird. Ohh well. I attributed it to "Smoother Aceleration and need for ECU to learn about new exhaust flow."
So, we drove some Saturday, Sunday and about 50 Miles Monday. Today Cech Engine light showed up







. 
Would anyone has any idea why light came on, what it means and how do i fix it.
Help plz!!!









Eugene


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## Buttero J-Lo (Mar 5, 2003)

*Re: Downpipe = Check Engine Light? (25psi)*

Do you still have a cat?


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## pdoel (Mar 14, 2001)

*Re: Downpipe = Check Engine Light? (ButteroJ)*

I had to have an O2 sensor replaced about 6 months after having my exhaust put on. It's been fine since (a year later now).
I've heard that often, when changing exhausts, if there's a lot of grease, etc., on the installers hands when he handles the O2 sensors, it can cause them to go bad prematurely.
Not sure how true that is, but it could be something like that. Or, it could be the O2 sensor was damaged in another way, during the install.
Or, it could be something completely different all together.
Heh heh.


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## 25psi (May 7, 2003)

*Re: Downpipe = Check Engine Light? (ButteroJ)*

Yes it is downpipe with cat.
Eugene


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## gt2437 (Jul 29, 2000)

*Re: Downpipe = Check Engine Light? (25psi)*

hi flow cat's can throw an efficiency code


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## 25psi (May 7, 2003)

*Re: Downpipe = Check Engine Light? (gt2437)*

How do i fix that?
Eugene


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## v-dub-u (Jul 28, 2003)

*Re: Downpipe = Check Engine Light? (25psi)*

bump would like to know as well. 
I have the same problem. I will be taking my car to the shop/dealer Saturday to see if they can fix the problem. I've heard a couple of suggestions so far. Could be the MAF sending some bad readings. Could be that CAT was not hat/warm enough ie accelrataing from low RPM's in 4th or 5th gear will pass alot of air through the cat but will not be hot enough for the CAT.

2 Faults Found:
16804 - Catalyst System: Bank 1: Efficiency Below Threshold
P0420 - 35-10 - - - Intermittent
16486 - Mass Air Flow Sensor (G70): Signal too Low
P0102 - 35-00 - -
Readiness: 0000 0000


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## gt2437 (Jul 29, 2000)

*Re: Downpipe = Check Engine Light? (v-dub-u)*

yes the efficiency code is thrown from the cat not getting hot enough. The 02 sensors read temperatures and which can be viewed via VAG Com.
I saw the efficiency code on my K04/Forced Motion 3" turbo back setup the following morning after driving 1500 miles the previous day. 
The code is harmless and in no way affects engine performance. 
Couple of ways that I know of that will prevent this from occuring:
a) drive with your foot in it more often







(which is not always practical or even possible depending on conditions)
b) look for a higher cell cat (300 -400 cell IIRC) that will generate higher temps the second 02 sensor is looking for. Usually the high flow catalytic convertors that do cause the CEL are only 100 cell.


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## 25psi (May 7, 2003)

*Re: Downpipe = Check Engine Light? (gt2437)*

What did YOU do?
Eugene


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## v-dub-u (Jul 28, 2003)

*Re: Downpipe = Check Engine Light? (25psi)*

in the morning when my car's been sitting over night it does not want to start. It almost stalls when the rpms drop to 2-300 then studders before regainig power and the RPM's jump back up. This has only been happening since I got the problem with the CEL's. any ideas?


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## paultakeda (May 18, 2002)

*Re: Downpipe = Check Engine Light? (gt2437)*


_Quote, originally posted by *gt2437* »_
b) look for a higher cell cat (300 -400 cell IIRC) that will generate higher temps the second 02 sensor is looking for. Usually the high flow catalytic convertors that do cause the CEL are only 100 cell.

Then what would be the point of a new cat?
As for the car... sounds like a bad MAF.


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## gt2437 (Jul 29, 2000)

*Re: Downpipe = Check Engine Light? (25psi)*


_Quote, originally posted by *25psi* »_What did YOU do?
Eugene

It only happened once for me, but I have since swapped out exhaust systems.


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## gt2437 (Jul 29, 2000)

*Re: Downpipe = Check Engine Light? (paultakeda)*


_Quote, originally posted by *paultakeda* »_
Then what would be the point of a new cat?
As for the car... sounds like a bad MAF.

Get one that actually does what it is supposed to do. There is a reason the stock cat is also ~300 cell. The cheaper cats are 100 cell and that's where problems happen.
The original poster does not have bad MAF, sounds like the efficiency code. v-dub-u most likely does, with the MAF code posted in there.


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## 25psi (May 7, 2003)

*Re: Downpipe = Check Engine Light? (gt2437)*

Well, took it to Autozone. Had dude there check it out. He was not too friendly so he killed his little scanner b4 i got a chace to write down that code. But he told me that it is CAT uneficiency.
So, that is it, right? Rear CO2 sensor. Is there a SURE fix for that to have that light go away?
Thanks.
Eugene


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## v-dub-u (Jul 28, 2003)

*Re: Downpipe = Check Engine Light? (gt2437)*


_Quote, originally posted by *gt2437* »_
The original poster does not have bad MAF, sounds like the efficiency code. v-dub-u most likely does, with the MAF code posted in there.


Thanks http://****************.com/smile/emthup.gif 
I'll let you guys know what the outcome is after I go to the shop on Saturday


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## v-dub-u (Jul 28, 2003)

*Re: Downpipe = Check Engine Light? (v-dub-u)*

Went to the dealer today and they replaced my MAF no more problems.


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