I've been dealing with a P0420 for several years new, but now it's starting to effect performance and gas mileage. Mileage is down from about 28 average to 24. Car is noticeably slower accelerating now than it was before. I now have a P1556 - Charge Pressure Control: Negative Deviation. I ran a log and at peak boost, actual boost is 1-2 psi lower than specified boost.
That being said, I think it's time to replace the converter. Those of you who have replaced them: what brand did you use, and how was the quality/fitment? I don't want to spend $800 on an OEM one, but I don't want to cheap out and get one that fits and performs badly.
Assuming you have a B5.5, it should be the same PN as B6 Audi 1.8t. Those pop up on Audizine B6 Classifieds fairly often, with relatively low mileage. Used of course, but original VW/Audi. I got mine there, had 20k miles on it, it was around $100.
I just recently installed my 034 high flow cat at work and it's great. does have some rasp but nothing crazy, fitment was perfect, currently using a 2" spacer. Only problem with it is I'm getting stored codes for no activity, low activity and cat inefficiency on my rear O2 sensor. Kinda annoying but in the process of sorting it all out.
After spending all day trying to wrestle the stock cat and downpipe out of the engine compartment with no success, a used stock cat is out of the question. I had looked at a couple of the high flow cats, but I don't think they will eliminate the P0420. 034 recommends a O2 spacer to avoid the code and I don't really feel like messing with that. If I'm replacing this thing, I want no codes.
I ended up ordering a Bosal cat and downpipe this morning. I'm going to have to cut the downpipe to get it out, plus the flex joint is torn badly.
The 034 cat does need a rear o2 spacer which all it does is move the rear o2 sensor out of direct exhaust flow. with the HFC there is more room to access the rear o2 sensor in case you have to swap spacers. I personally went with the 034 unit only because I'm tuned have a full 2.5" exhaustalong with some other things and wasn't going to drop a ton of money on a stock unit.
I used a Bosal replacement in one of my 1.8t's, I'm not a performance guy, just want it to run reliably. If the flex pipe is shot just cut it out and the cat/down pipe will come out a lot easier. It's like a Chinese puzzle getting it back in because you need to connect the downpipe to the cat before you put it back in the car. The only other issue is sometimes an aftermarket downpipe has the bracket either in the wrong place or just sits in a weird position and doesn't line up with that bracket underneath the car so you may need to McGyver it to connect.
I got the cat replaced today. I used a bosal 099-3021 converter with the 860-975 downpipe for the auto trans. There was no way to get the stock converter and downpipe out without cutting the pipe in half, right before the flex joint. My outer flex layer was torn, but not leaking. I was able to get the new cat and pipe installed as a unit from the top. There's just not enough room to bolt them together when they are in the car. Plus the holes in the flanges where the cat and the downpipe are so oversized (over 12mm, designed for a 8mm bolt) you could never get them aligned correctly. I also installed the rear O2 after the cat was installed. The downpipe fit OK for the most part. The mounting bracket lined up nicely, but the pipe itself is 53mm instead of 55mm. The bosal pipe coupler that I got is a piece of crap. I had to really tighten down on it to try to get it to seal on the smaller pipe, but instead it distorted and leaked terrible. I had to re-use my stock rusty piece and only have the tiniest of leaks.
I'm a little concerned about the lack of a heat shield on this cat. I was getting all sorts of terrible smells from the new exhaust pieces, hopefully it's just the grease and oils burning off and not some part of the car melting.
when I installed my cat back and recently my high flow cat I did notice a weird smell from the exhaust but the most noticeable was from when I installed the cat back, The smell lingered for a few days but eventually went away completley
I did a write-up with photos for replacing the downpipe and cat converter on a '99 1.8T a while back. I replaced both the downpipe and cat converter with ones from Bosal.
The smell from the new cat and downpipe was probably just the adhesive burning off from the labels that were glued to the parts or possibly the labels themselves if you didn't remove them prior to installation.
I read through your write-up at least 10 times beforehand, so thanks for that. I am a little annoyed at how easy it is to remove the cat and downpipe on the manual cars compared to the autos.
I made sure to clean all the parts thoroughly before installation with carb cleaner. Years ago, I forgot to remove a sticker off a exhaust pipe before installation and was smelling that thing burning off for months.
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