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    Exclamation P0420 Code

    I know there are threads on this, but I feel that a lot of situations can be unique. I have a 2004 w/ 1.8T AWM. It has 72,000 miles, and I just bought it 1 month/2000 miles ago. I've done motor mounts, VCG, cleaned MAF, cleaned Throttle Body and other items myself, so i feel that I am capable of doing some work myself to get rid of this code.

    I have noticed lately that my gas mileage has been TERRIBLE. most of my recent short trips (City of Chicago, 5-10 miles each) have been netting me in the range of 10-12 mpg by the time I get back (My wife says that her longer trips including highway have been netting 27-28 mpg though). I don't have a CEL, but I thought that something wasn't right with this. So I took it to Autozone and had them scan using their ODB-II tool. It threw two codes:

    P0102 - Mass or volume air flow sensor circuit low input
    P0420 - Catalyst system efficieny below threshold bank 1

    Now, the P0102 is probably my fault, because I had unplugged the MAF sensor to see if it ran better/worse (saw that in some threads) recently, but plugged it back in. In hindsight, probably should have waited until after getting the codes checked.

    The P0420 worries me a bit though, especially in conjunction with the terrible gas mileage lately.

    What is the first thing that I should check/change to see if I can fix this problem?

    Thanks for everyone's help!

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    i have p0420 and was told i need 2 new catalatic converters

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    Quote Originally Posted by Kaaaaal View Post
    i have p0420 and was told i need 2 new catalatic converters
    Hmmm. I know the explanation for p0420 read "faulty catalyst." But who told you that you needed 2 of them? There is only 1 on the car.

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    Moderate Moderator Steve in Chicago's Avatar
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    There is one CAT on the 1.8t and there are 2 (or 4) on the V6.
    The trouble code number will indicate weather the problem is on bank 1 (passenger side) or bank 2 (drivers side).

    Some specific models and years (I don't remember which) have a CAT monitoring system that is overly sensitive. This will get you CAT efficiency codes when you really shouldn't be getting them. If your car is one of these, the dealer should be able to re-flash the software in your ECM for free under one of the recalls. Do some searches to see which cars this applies to.

    16804/P0420/001056 - Catalyst System; Bank 1: Efficiency Below Threshold
    Possible Causes

    * Leakage in Intake and/or Exhaust System
    * Catalyst faulty
    * Oxygen Sensor(s) faulty
    * Oxygen Sensor(s) Control faulty

    Possible Solutions

    * Check Intake and Exhaust System for Leaks
    * Check Catalyst
    * Check Oxygen Sensor(s)
    * Check Oxygen Sensor(s) Control
    o Perform Oxygen Sensor(s) Aging Check

  6. #5
    Moderate Moderator Steve in Chicago's Avatar
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    dbeckwit, you need to have the codes cleared then drive the car a bit then re-scan. The P0102 may have been generated when you unplugged the MAF, or you may have a MAF problem. It'd be interesting to see what your learned fuel trims are showing. Where in Chicagoland are you? I'm in the city and have vag-com. I'd be happy to take a peek at your car and do some scans.

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    Im out of town for the weekend, but will have the codes rescanned and cleared on monday, autozone can do that with their obd 2, right?

    I live in the city, lakeview neighborhood, wellington and racine. That would be awesome if we could meet up next week sometime for you to help me out. Where are you located?

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    Moderate Moderator Steve in Chicago's Avatar
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    I PM'd you my contact info.

    AutoZone will scan but from what I was told, they are not supposed to clear codes. I imagine if you get the right salesperson, they may do it anyhow.

    On second thought, don't bother with AutoZone clearing codes. Clearing codes also clears the learned fuel trims. I'd like to see what those numbers are showing before they get erased.

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    Mine was a faulty 02 sensor (actually, I just needed to space it out due to the high flow cat, but you get the point...it wasn't the cat).

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    Steve in Chicago, thank you for the clarification. i didn't know the V6 had two of them.

    swcrow, I'm hoping that's the issue, O2 sensor.

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    UPDATE (sorry for length)

    I cleared the code (P0420) 3 separate times, and it would just pop right back up next time I scanned. Since my car is a 2004 with only 73,000 miles, I fall under the federal emissions warranty to have my failing catalytic converter replaced by VW.

    I took it in to VW Autobarn in Chicago, and had them test it. After hour and a half, guy says: unfortunately, they cannot change it out under warranty right now. They ran diagnostics and such, no check engine light came up. Then they did an emissions test, and the cat did not fail. Since it did not fail, they cannot replace it. This seemed weird to me, but wasn't much I could do, so I went home.

    When I got home, finally figured out how to run some O2 sensor and catalytic converter tests using my VAG-COM (which I will try to do a write-up for soon). They showed that both the pre and post cat O2 sensors are working fine. Then I ran the cat test, and it showed, "CAT B1 not OK". This was the definitive evidence i needed to show that VW was acting funny?

    I called them and spoke with a guy named John. He promised he'd talk to the tech and get back to me. 2 days later, got a call from him. Turns out, the work notes say "too hot to test" on them, so they didn't run an emissions test? Also, he said that all they need to replace the cat under warranty is to clear the codes, and then have the codes pop back up when the scan again.

    So i will be taking the car in sometime in the next week or so, and get the cat replaced for free under warranty. Would suggest all that fit the parameters, try this out.

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    I have a question regarding this fault code and the Check Engine Light. In the description for P0420, it says Lower Limit Exceeded - MIL ON. This should indicate that I've got a check engine light, correct?

    I've not had a check engine light yet, which worries me. Is it easy for someone to have unplugged that bulb? Or is it common for it to have burnt out?

    TIA

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    Moderate Moderator Steve in Chicago's Avatar
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    Does the CEL light up when you first put the key in the ignition and switch it to ON?

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    Quote Originally Posted by Steve in Chicago View Post
    Does the CEL light up when you first put the key in the ignition and switch it to ON?
    At work, so I don't have the car in from of me, but No I don't believe it does.

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    Quote Originally Posted by swcrow17 View Post
    Mine was a faulty 02 sensor (actually, I just needed to space it out due to the high flow cat, but you get the point...it wasn't the cat).
    Actually, it is the cat.

    High flow cats obviously do not filter as much and the exhaust that comes out does not meet standards or the placement of the hole for the rear 02 is not optimal. you are forced to put space between the exhaust gas and the 02 sensor. Nothing wrong with the sensor, its doing its job.

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    Quote Originally Posted by dbeckwit View Post
    I cleared the code (P0420) 3 separate times, and it would just pop right back up next time I scanned. Since my car is a 2004 with only 73,000 miles, I fall under the federal emissions warranty to have my failing catalytic converter replaced by VW.

    I took it in to VW Autobarn in Chicago, and had them test it. After hour and a half, guy says: unfortunately, they cannot change it out under warranty right now. They ran diagnostics and such, no check engine light came up. Then they did an emissions test, and the cat did not fail. Since it did not fail, they cannot replace it. This seemed weird to me, but wasn't much I could do, so I went home.

    When I got home, finally figured out how to run some O2 sensor and catalytic converter tests using my VAG-COM (which I will try to do a write-up for soon). They showed that both the pre and post cat O2 sensors are working fine. Then I ran the cat test, and it showed, "CAT B1 not OK". This was the definitive evidence i needed to show that VW was acting funny?

    I called them and spoke with a guy named John. He promised he'd talk to the tech and get back to me. 2 days later, got a call from him. Turns out, the work notes say "too hot to test" on them, so they didn't run an emissions test? Also, he said that all they need to replace the cat under warranty is to clear the codes, and then have the codes pop back up when the scan again.

    So i will be taking the car in sometime in the next week or so, and get the cat replaced for free under warranty. Would suggest all that fit the parameters, try this out.

    Good News! Dropped the car off this morning, and just got a call that it is done and sending the wife to pick it up. I tested last night to make sure the P0420 was still popping up. They replaced the catalytic converter under the 8 year/80,000 mile federal emissions warranty. Nothing like getting a ~$800 part and nearly $300 in labor for free!


    Will be looking more into the CEL when I get home this evening.

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    Quote Originally Posted by Toronto5.5 View Post
    Actually, it is the cat.

    High flow cats obviously do not filter as much and the exhaust that comes out does not meet standards or the placement of the hole for the rear 02 is not optimal. you are forced to put space between the exhaust gas and the 02 sensor. Nothing wrong with the sensor, its doing its job.
    You are correct, I still have the same o2 sensor and it works correctly, I just worded that statement wrongly

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    Quote Originally Posted by Steve in Chicago View Post
    Does the CEL light up when you first put the key in the ignition and switch it to ON?
    Well, surprise surprise, no the CEL does not light up when first turning key. This upsets me a bit because without your help in scanning my car Steve, I would not have known about the CAT problem for a while (potentially until after the 8/80,000 warranty.)

    No black electric tape over it, so it either burned out (not likely from what I hear) or previous owner or car lot removed LED..... Shady business.

    When i send cluster in to Paul at pixelfix for the speedo gauge motor fix, i'll have him figure out/fix the CEL issue.


    Side note, when I got home with the car, i realized that they had not put the screws back on that hold the fender liners to the belly pan. Frustrating, because it was ~75-90 minutes there and back to have them replace the parts that they lost... oi vey

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    Moderate Moderator Steve in Chicago's Avatar
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    I wonder if someone opened the cluster to clip the CEL light and in the process messed up the gauge that is acting funny.

    I bet 90% of the people wouldn't have noticed the missing fender liner fasteners. I HATE lazy work like that.

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    Quote Originally Posted by Steve in Chicago View Post
    I wonder if someone opened the cluster to clip the CEL light and in the process messed up the gauge that is acting funny.

    I bet 90% of the people wouldn't have noticed the missing fender liner fasteners. I HATE lazy work like that.
    wow, I could definitely see that being the case. damn.

    agreed, hate lazy work as well. glad I noticed, because I had to replace the fender liners when I bought the car due to no screws on either side there. the liners flapped at high speed against the tires and eventually disintegrated where they had touched.

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    Quote Originally Posted by dbeckwit View Post
    wow, I could definitely see that being the case. damn.

    agreed, hate lazy work as well. glad I noticed, because I had to replace the fender liners when I bought the car due to no screws on either side there. the liners flapped at high speed against the tires and eventually disintegrated where they had touched.
    wow. was under the car today and found another really lazy product of VW autobarn chicago. the bar that crosses the exhaust at the midpoint of the car, was only bolted back on on one side. was hanging from the car, bout 4-5 inches below the belly of the car. other bolt was flighty threaded in where it was supposed to be. don't think I'm going back there anytime soon.

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    Moderate Moderator Steve in Chicago's Avatar
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    Doesn't that just pizz you off!? Seriously, what is up with that sort of service? You should let a service manager over at AutoBarn know. In a perfect world, he'd go talk to that technician about the quality of his work. I suppose in the real world, you may just be wasting your own time.

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    Quote Originally Posted by Steve in Chicago View Post
    Doesn't that just pizz you off!? Seriously, what is up with that sort of service? You should let a service manager over at AutoBarn know. In a perfect world, he'd go talk to that technician about the quality of his work. I suppose in the real world, you may just be wasting your own time.
    I called today and let the service manager know. He was surprised to hear, as he said it was a SENIOR tech that did the work. Thanks for the idea of calling in.

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    Needed to space te 02? Like throw am extra washer on it?

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    Didn't see a definitive fix listed in this post but I guess the new Cat solved the problem? I've searched a ton of P0420 threads here and on Vortex and rather than start a new one I'd thought I'd post my deets in here. Ultimately I guess I'm going to need a VSDS to read the O2 data but until I get one, or in touch with someone with one I was hoping gather some more info to help TS. In my case, 2003 AWM (I think) 1.8t Passat GLS I was running a straight catless pipe for about 18 months, (big mistake but it's done) I don't need to do emission for another 6 months but thought I'd put a cat back in early and work out any bugs pre test. Anyhoo I got the wrong Cat 98-02 Passat/Audi instead of 03-? but figured it would work fine so had it installed. Got immediate MAF and misfire codes due to a pulled pin on MAF connector. I'd run it only about 20 miles before fixing it but there was some hard acceleration and 70-80 mph highway speeds with some misfires and pulsing under hi load. With the pin fixed the MAF codes cleared and the car runs great, real smooth, but but after 30-40 miles I'm now getting the P0420 code. I was getting this code when I first put in the straight pipe but made it go away with a restrictor on the O2 sensor take off. I would prefer not to go that route this time, if it's even the fix. I currently do hear a pulsing exhaust leak under hard acceleration but I think it's the flex pipe after the Cat (I'll know better when the rain lets up) but otherwise the engine runs and sounds nice. I have an Android app (ScanMaster Lite) with an ELM327 bluetooth interface to read and clear does but not too much functionality beyond that.
    Anyhoo any thoughts out there as to 1) Is the the 98-02 Cat not compatible, it did take some pipe work mods due to it being slightly longer in length but I don't hear or feel any leaks and the honey comb was good? 2) Did running Catless for 25,000 miles change anything in the system? 3) Did running 20 miles with the MAF induced misfires blow a hole in the replacement cat? Or is it maybe something else? Any thoughts would be appreciated.
    Also FWIW ScanMaster Lite throws a bunch of I/M Readiness "Not Passeds" after I clear the P0420 but after about 20-30 miles when the MIL code comes back all of the available readiness codes go to green???

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    Anyone? Anything?

    So I got some VCDS info but being totally new to it with a borrowed tester I don't know if the data in the .PDF is any good at all. If anyone has suggestions as to what parameters to log ect please let me know.
    Attached Files Attached Files
    Last edited by BartBart; 10-28-2013 at 03:25 PM. Reason: fix attachement.

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