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2003 Cyber Green 2.0
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Discussion Starter · #1 ·
I've been struggling with the P0420 code with my 2003 New Beetle for about a month and a half. There are no drivability problems, and highway mileage is at a respectable 28MPG. I scanned it with a generic OBD2 reader and sure enough, it's the dreaded "Catalyst Efficiency Below Threshold" code.

I'm using the Android app, Torque. Clearing the code will set my emissions readiness monitors to "Incomplete" for the O2 Sensor and EVAP monitors but turn off the "Check Engine" light. After 1-2 weeks of driving, those tests will complete and the P0420 "Check Engine" light will return. The Self-Test in Torque will show a fail for either:

"Rich to Lean sensor threshold voltage (constant"
Main: 3,686 Current: 3,171

"Rich To Lean Sensor Switch Time (calculated)"
Min: 580 Current: 1,500

Asked my mechanic about it and they scanned it with a VAG-COM. He told me my O2 sensors are fine and my catalytic converter would need to be replaced to resolve the code.

Anyone successfully tackled a P0420 code and what did it take? Changing the O2 Sensor, the Cat, or another remedy like using Seafoam to "clean" the existing cat?

Thanks.
 

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I can go on all day about P0420.

1 - you WILL NOT be able to clean the converter with SeaFoam, get this out of your mind now.

2 - Replacing O2 sensors likely will not improve things.

3 - Some of the catalytic converter had an extended warranty, but you may be outside of even the warranty extension.

4 - You can use aftermarket converters with good results, many are under $250 for the converter with a downpipe.

5 - Check the engine temperature to make sure it is not running too cold. Often P0420 can be caused due to cool running engine temperatures and carbon build up in the converter. Engine should run at around 205F consistently even when the outside temp drops below 70F.

6 - Since you have Torque, if you can figure out how to set up Logging, you can gather a 4 minute warm idle Log that includes ALL parameters that are available and we can look at the data and graph the O2 sensors and see how the converter is behaving.

7 - Make sure you have no exhaust leaks, leaking flex pipes, cracked O2 sensor bung welds and post cat exhaust leaks can cause P0420 to trigger as well.

8 - Post the Freeze Frame info for when the P0420 occurs.

See this graph for what a good/new catalytic converter looks like when graphing the Post-cat O2 sensor at a steady highway speed - http://newbeetle.org/forums/questio...etle/114618-emissions-help-8.html#post1279169
 

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dreaded p0420

It's worth a try before you spend 300 plus on a new CAT.

I did and tackled that P0420 for a year or two and was getting ready to change out the CAT when I decided to try the Seafoam cleaning and it helped clear the P0420 code and has never come back again. Now I clean it once a year. I have the 1.8t so I'm not sure how you would clean your 2.0. I guess just slowely let the intake suck it in. Worth a try I'd rather spend $8 bucks than 300 bucks for a new CAT if you're successful. Good luck.

That is correct I believe it's due to Carbon build up on the CAT or O2 sensors so Seafoam is the best carbon cleaner I have used so far.
 

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Proper engine coolant temperature is also a must, once the engine temp drops by 10-15+F the engine will start to run richer, causing excessive carbon build up in the combustion chambers as well as the exhaust.
 

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2003 Cyber Green 2.0
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Discussion Starter · #5 ·
It's worth a try before you spend 300 plus on a new CAT.

I did and tackled that P0420 for a year or two and was getting ready to change out the CAT when I decided to try the Seafoam cleaning and it helped clear the P0420 code and has never come back again. Now I clean it once a year. I have the 1.8t so I'm not sure how you would clean your 2.0. I guess just slowely let the intake suck it in. Worth a try I'd rather spend $8 bucks than 300 bucks for a new CAT if you're successful. Good luck.

That is correct I believe it's due to Carbon build up on the CAT or O2 sensors so Seafoam is the best carbon cleaner I have used so far.
Thanks for sharing your story. I was curious to hear about people who had success doing that. Which intake do you apply the Seafoam and do you need any special tools? Do you have a guide how to do it by any chance?
 

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2003 Cyber Green 2.0
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Discussion Starter · #6 ·
Proper engine coolant temperature is also a must, once the engine temp drops by 10-15+F the engine will start to run richer, causing excessive carbon build up in the combustion chambers as well as the exhaust.
What's the ideal coolant temperature I should be looking for? Last time I scanned it with Torque after a short drive, it was at 92C.
Appreciate all the other information you've posted. I was thinking to replace my cat eventually, but closer to when my emissions test is due. I wouldn't want to put it an aftermarket cat and have the P0420 come back before I need to renew my sticker.
 

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See #5 in my comments above. Engine needs to run very close to 205F even while cruising on the highway when the outside temps are below 70F.

I have a decent aftermarket converter on my 1.8t that I have been running for about 15k miles without a problem.

I replaced it not because of P0420, VW replaced it earlier for P0420, but the replacement converter had a marble size piece of monolith dancing around inside the converter making noise.

I think the replacement converter was dropped in shipping before it was installed on the and part of the monolith broke loose.
 

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google

Thanks for sharing your story. I was curious to hear about people who had success doing that. Which intake do you apply the Seafoam and do you need any special tools? Do you have a guide how to do it by any chance?
Google this topic (how to seafoam vw 1.8t jetta) and it'll show you a small Utube video of someone doing their car just to give you an Idea of what's required. I know you have the 2.0 so yours might be done using the brake booster line or any vacuum line going strait into the Intake. Use a tapered cut clear tubing to only suck in a little at a time. Remember that when you apply the seafoam it's gonna throw a few engine codes but those will clear right away or as required by the ECM.
Remember only do 4-5 ounces.
let us know how it turns out.
 
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