# smog and suck p0420 code, exhausted options



## FamousEric (Jul 17, 2007)

so here's the deal...eff smog.
UPDATE: all codes are gone and here's what i did..read further if you want to know the pain i went through.
p0420: new oxygen sensors, "help" spark arrester, new air filter, cleaned old MAF, NEW COOLANT TEMP SENSOR
p0171: went away after NEW SPARK PLUGS
p0304: went away after NEW SPARK PLUGS
p1142: NEW MAF
p1152: NEW MAF
all parts are new OE VW...trust me, don't settle for saving $2 or $3 on plugs from autozone etc..
I've done everything i can to try to get my car to pass smog. it's a 2001 jetta 2.0 Auto 105k miles. i've done the following (not in any order):
cleaned the MAF
changed plugs/wires
replaced both o2 sensors
oil change
02 sensor mod
pulled the engine cover off and went to town on the vacuum lines with carb cleaner
replaced air filter
after i replaced the air filter, oxygen sensors, plugs/wires, and did an oil change i had a friend use his vagcom to clear the original p0420 code. i was able to run the car for about 20-30 miles, turned it off, and back on..the CEL popped back up. still p0420. went a few days researching what it could be and i had some left over carb cleaner from checking another car for vacuum leaks, so i did the same with the jetta. no vacuum leaks. i reset the ecu again by pulling the battery cable and drove it for about 150 miles - around town, freeway, etc without shutting it off, then parked it at home, shut it off, ran inside to grab my cell phone, came back out and started it up...CEL is back.
So i found a guy that would do everything he could to help me pass smog and i took the car to him with the CEL still on, he ran the diagnostics and tailpipe test and he said it was running cleaner than 80% of the cars he tests so the cat's doing its job. so he cleared the code for me and of course the car isn't "ready" for smog, so he told me to go drive it and bring it back occasionally until it shows ready...after about 120 miles and not shutting the car off, the car still isn't showing "ready." so i told him that if i shut the car off and back on, i know the CEL will pop back up..."give it a try" didn't pop back up, and still not ready. i had to go back to work at this point so i drove away.. 2 minutes in the the drive the CEL pops back up.
i started lurking other threads and found that some turbo people have the same problem (i did with my sti in the past) with the same CEL. by running an extention on the o2 sensor, the problem was alleviated. (tuning my sti resolved this problem)...the jetta went a couple DAYS of regular driving and then randomly the CEL popped back up...still p0420.
so another thing i found was people having a bad/dirty maf that was potentially causing this code. i bought some maf cleaner and did work on the stock maf...the car idle'd a little bit better (not that it was really bad to start off) but didn't resolve my problem.


_Modified by FamousEric at 6:24 AM 9-4-2009_


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## Cheetotastic (Sep 29, 2008)

*Re: smog and suck p0420 code, exhausted options (FamousEric)*

My friends ford ranger had this same code and I did some research and we thought we would have to buy a new cat. After a couple weeks the code went away and i asked what he was doing when the code went away. He said he was towing his boat up a hill. I remember reading that sometimes the cat will have some carbon build up so if you work your engine hard long enough it will burn it out. If you are desperate enough you could try rolling down windows, cranking up the AC, and going up some hills or anything else you think could run the engine harder. Hopefully you'll have a lucky day and the light will go off.


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## FamousEric (Jul 17, 2007)

to be quite honest, i drive it pretty hard at times (when i'm passing w/ac on in 105* CA weather). since the code stores, i don't think it'll just go away.
when i was being helped by the smog tester, he told me (at one point) that the light wasn't on and that he could test me "right now" and it'd pass...but since the light on the dash was on he'd have to fail me for visual...


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## FamousEric (Jul 17, 2007)

is it possible to get this car chipped and to possibly eliminate a future p0420??


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## FamousEric (Jul 17, 2007)

bought a coolant temp sensor in hopes that for some reason engine temps make that much of a difference on the a/f mixture and exhaust gasses in the cat.


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## seanmor (Dec 23, 2004)

*Re: (FamousEric)*

I also have a 2001 Jetta 2.0 with this code and I have been working on it for a few months. With a VAGCOM, you can set the readiness codes in about 10 minutes or less without driving the car. This should allow you to get the car smogged before the p0420 code comes back. Here is a link to the procedure:
http://forums.fourtitude.com/z...74940


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## FamousEric (Jul 17, 2007)

much appreciated!


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## Oranrado (Sep 9, 2005)

I'm getting the same code with a leaky manifold.. You might want to try a low pressure smoke test at a garage... you might have a small crack somewhere between the 2 O2 sensors causing the readings to be too different... Just a thought... good luck!


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## seanmor (Dec 23, 2004)

*Re: (seanmor)*

Update: the P0420 code is gone!!! I did the re-wire of the ground for the coil pack that was described in this thread: http://forums.fourtitude.com/zerothread?id=2382004 . I just used some spare wire I had lying around, cut the brown wire going to terminal 1 on the coil pack wire harness, soldered the new wire to the harness end of the brown wire, and connected the other end to a ground connection under the battery tray. Once all that was done, I cleared the dtc's and set the readiness. That was last sunday and I have been checking for dtc's every night since but so far nothing. Also, the car feels less sluggish and fuel economy has increased 4 mpg or so.


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## blacksparx (Aug 5, 2009)

*Re: (seanmor)*

I just did the same coil ground rewire this weekend but instead of cutting the brown ground wire I spliced into it near the harness and ran a ground to the post under the battery tray. I hope that clears the ground CEL problem. If not I will have to cut and solder.


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## seanmor (Dec 23, 2004)

*Re: (blacksparx)*

P0420 code came back







. I was hoping for a miracle.. I think I give up trying to figure this one out. The car still passes readiness checks so as long as I can pass emissions, it should be ok. 
The codes related to the coil however did not come back. So it looks like that one is fixed with re-wiring the ground http://****************.com/smile/emthup.gif


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## bearing01 (May 27, 2004)

*Re: smog and suck p0420 code, exhausted options (FamousEric)*


_Quote, originally posted by *FamousEric* »_
I've done everything i can to try to get my girlfriends car to pass smog. it's a 2001 jetta 2.0 Auto 105k miles. i've done the following (not in any order):
02 sensor mod

1) Explain this.
2) Did you inspect for an exhaust manifold or down pipe leak? Any leak between the engine and the cat converter?
3) How many miles on the O2 sensors?
4) Do you have smog report numbers for HC, CO, CO2, NOx, O2 at idle and cruise?


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## FamousEric (Jul 17, 2007)

*Re: smog and suck p0420 code, exhausted options (bearing01)*


_Quote, originally posted by *bearing01* »_
1) Explain this.
2) Did you inspect for an exhaust manifold or down pipe leak? Any leak between the engine and the cat converter?
3) How many miles on the O2 sensors?
4) Do you have smog report numbers for HC, CO, CO2, NOx, O2 at idle and cruise?

1. you're thick if you didn't read my whole post, i explained everything that i've done to try to fix it
2. yes, i'm a stickler for exhaust leaks, if i hear one, i go nuts.
3. brand new
4. all numbers are well below limits..it'd pass if i didn't have a CEL on.


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## seanmor (Dec 23, 2004)

*Re: smog and suck p0420 code, exhausted options (FamousEric)*

Were you able to set the readiness codes with vag com?


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## FamousEric (Jul 17, 2007)

no







i seriously give up.


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## bearing01 (May 27, 2004)

*Re: (FamousEric)*

http://www.p0420.com/
It means you have too much activity on the O2 sensor after the catalytic converter. A well tuned car can often pass the smog numbers even with minimal catalytic converter help. High downstream O2 sensor activity means the cat is not helping clean up the exhaust much, and is observing the same alternating voltage as the pre-cat sensor. That is, fluctuation in air/fuel ratio due to EFI fuel trim is observed at the cat output. The cat therefore is not absorbing the O2 in the exhaust as it should. The Cat for some reason may be damaged and needs to be replaced.
I figured you could probably see a higher than normal O2% number on the smog results. While O2% is not a smog number, on a 5 gas analyzer it will help tell what the problem is. You didn't provide that number so we don't know for sure what's going on. You should take the car to someone who has an oscilloscope, who can look at the down stream O2 sensor voltage waveform to compare it to the upstream O2 sensor waveform. If they are both the same and the Cat is good & hot then you'll likely have to replace the cat.


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## FamousEric (Jul 17, 2007)

fixed p0420 with a new coolant temp sensor.
i then had 2 other codes pop up; misfire cylinder 4, and running too lean (cant remember the codes but i think one was p1152)..replaced the plugs with VW plugs then.. - p1142 and 1152 (ross-wiki said common denominator on both was maf). i bought a new maf sensor and cleared the codes... no CEL's and passed smog yesterday.
Thank God.


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## FamousEric (Jul 17, 2007)

oh...and 2 different vag's, 2 different people, neither able to set readiness.


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## FamousEric (Jul 17, 2007)

*Re: (bearing01)*


_Quote, originally posted by *bearing01* »_http://www.p0420.com/
It means you have too much activity on the O2 sensor after the catalytic converter. A well tuned car can often pass the smog numbers even with minimal catalytic converter help. High downstream O2 sensor activity means the cat is not helping clean up the exhaust much, and is observing the same alternating voltage as the pre-cat sensor. That is, fluctuation in air/fuel ratio due to EFI fuel trim is observed at the cat output. The cat therefore is not absorbing the O2 in the exhaust as it should. The Cat for some reason may be damaged and needs to be replaced.
I figured you could probably see a higher than normal O2% number on the smog results. While O2% is not a smog number, on a 5 gas analyzer it will help tell what the problem is. You didn't provide that number so we don't know for sure what's going on. You should take the car to someone who has an oscilloscope, who can look at the down stream O2 sensor voltage waveform to compare it to the upstream O2 sensor waveform. If they are both the same and the Cat is good & hot then you'll likely have to replace the cat.

thanks for the reply, though the cat didn't end up being the problem. i passed yesterday and everything read waaaaay below limits. i even kept the o2 sensor spacer in with thoughts that if i took it out i might get a lean code again..?? so even possibly running richer than it should, passed easily.


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## seanmor (Dec 23, 2004)

*Re: (FamousEric)*

Did you use the spark plug spacer or one of the special O2 spacers on Ebay?


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## FamousEric (Jul 17, 2007)

*Re: (seanmor)*


_Quote, originally posted by *seanmor* »_Did you use the spark plug spacer or one of the special O2 spacers on Ebay?

i used the taperred spark plug spacer from the "help" parts in kragen. autozone, napa, etc.. has them also. if you search vortex, somebody did a good write up on how to use it properly on 1.8t's throwing the p0420 code.


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## seanmor (Dec 23, 2004)

*Re: (FamousEric)*

This is great, I have been battling this 420 code for months now. I was expecting to have to replace the cat but I'll try this first.
Thanks!


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## FamousEric (Jul 17, 2007)

*Re: (seanmor)*


_Quote, originally posted by *seanmor* »_
This is great, I have been battling this 420 code for months now. I was expecting to have to replace the cat but I'll try this first.
Thanks!

good luck!


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## BlackZach (Nov 4, 2007)

good luck last i went my car was to low so they just passed my car


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## dgausa (Dec 17, 2009)

*Did it fixed your problem*

I am having the same problem I just want to know if changing the temp sensor fixed your problem


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## FamousEric (Jul 17, 2007)

sorry, it's been a while and i know your post is old. i don't visit vortex that often. the new coolant temp sensor ended my problems. it was the cheapest part but the very last thing that i changed. if you haven't fixed yours, good luck to you. hope this thread helps others in need.


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