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Discussion Starter #1
I have read a lot of the threads about this code. I have a 2002 beetle 2.0 avh engine with 179,000 miles. This car runs perfect and idles perfect with good gas mileage. I got the code after I had the oil changed as well as the air filter. Could either of these cause the 420 code? If it ends up that I need a cat, what should I buy? The VW cats don't seem to be much good as shown by there extending warranties. A lot of people say aftermarket cats aren't good. I have a feeling VW dropped the ball with their software being to restrictive. Are there updates to the 2002 2.0 computers?
 

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corrrect

Correct there is a software update out there I just haven't gotten around to spending 100-150 for the Dealer to do that. Mine has been throwing that code for the last three years now and I'm glad I didn't replace the CAT cause that's not the problem. I would check all the components and sensors that correspond to emmisions systems before buying a new CAT.
 

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Not likely the catalytic converter.

Many times the P0420 code is due to degraded O2 sensors, minor exhaust leaks and possibly the Catalytic Converter efficiency thresholds being too tight.

Degraded O2 sensors are common. O2 sensors are consumable items at it is recommended they be replaced at 100k miles under normal conditions. Have your O2 sensors ever been replaced?

Exhaust leaks, these can be at junctions of pipes or end hairline cracks at welded joints, even the O2 sensor bungs welded onto the pipes. Keep in mind these cars have the flexible coupling at the exhaust manifold and usually a slip coupler after the Catalytic converter and at the resonator.

Lastly there is a ECU update. Most manufacturers have ECU updates for certain models that phantomly trigger the P0420 code.

If you do not have your own scan tool at this point, suggest you purchase one of you own. Read the OBD Scan Tool link below in my signature.

This may be easier and cheaper to manage the issue rather than to try and fully correct it. You should rule out the obvious issues, then decide the next step of your action.

My 2003 1.8t GLX had this code when the car was newer, probably less than 40k miles, dealer replaced the catalytic converter, not sure if they flashed the ECU?? Never had the P0420 code again, however, the replacement converter they installed had a piece of the monolithic element break and get caught between the first and 2nd converter beds, sounded like I had a bunch of marbles in my exhaust. I replaced the converter again with a decent aftermarket one for about $250 and have had no issues so far. But I replaced my converter not due to a code, but noise and concern my converter might degrade more.

If I had to do it all over again, I would not have let the dealer replace the original converter. Likely I would not have had to replace it. I think the replacement was dropped at one point cracking the monolithic cell???
 

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Discussion Starter #4
I don't know the maintenance history on this car so I don't know about the o2 sensors. Will the originals last 179,000 miles without needing changing? I believe the cat is an original. The previous owner had a bad coil so his car would not start. He didn't know that but he kept throwing parts into it and kept trying to start it. All that fuel went into the cat and sat there. That can't be good for it. am going to make an appointment at my dealer and have them run full diagnostic so I will know what is really wrong with the car.
 

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At 175k the O2 sensors definitely could use a refresh.

Raw gasoline in the Catalytic Converter by itself is not necessarily a problem. The gasoline does not hut the converter by itself, it is the extreme heat that can melt the ceramic monolith if raw gasoline keeps getting dumped into the converter and the car is continue to be driving.

Keep in mind the dealer will be a very expensive place to have your work performed. The only think I personally would
request the dealer to perform would be to reflash the ECU if there is an update that may correct the P0420 code.

If you have as many miles on the car at this point and did not have a P0420 code before, I would likely start with new O2 sensors, DIY or Indy shop. You can likely purchase Bosch sensors on Amazon for around $50 as long as they are not wideband sensor, which are usually more expensive.

What other problem(s) do you have with the car?
 

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I just realized you had create 2 posts regarding this issue.

You need to get your car inspected?

You need to check with your state to see if you can get your car inspected with any Emission Readiness Codes still not cleared. Many states allow up to 1 Emission Readiness Codes to not be cleared as long as the car does not have the CEL on.

Also the P0420 code likely only will trigger during highway driving. If you can get the Emission Readiness Codes to clear with very little highway driving, the only drive in town, you likely can get your car through the inspection process and address this issue on your own schedule.
 

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Discussion Starter #7
Sorry about the multiple threads, I'm new at this. The dealer told me he would charge $90 to find the problem. I agree with you regarding driving in town. I tried that for awhile but it wouldn't reset the computer. I may try doing the drive cycle I found on VW Vortex. I really want to try everything before I spend $90.00 on the dealer.
 

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I'd try to find a local independent VW shop over the dealer. Might be able to get some guidance on a shop here: VWVortex.com - New England

You can also find someone with VAG-COM to help you out. You can use it to look at sensor performance, etc. and really be able to troubleshoot things.

FWIW, when my cat failed, my mileage didn't decrease. I had a bunch of rattling though, as the innards of the converter just came apart.
 

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Discussion Starter #10
I do have a scan tool and I am good at reading the o2 sensors but the other functions I can't tell if they are good or not.
 

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Discussion Starter #11
I have been using my scan tool and I have discovered my running temperature is 185 which is a little low. I can't imagine I would get a P0420 code from that. Also the fuel trim is under 2%and the MAF sensor is good.
 

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Get the car on the highway in the evening and see if your temp holds at 185 or drops.

185 is about 20F too cool, this will make the car run slightly rich as well.

Get the temp sorted and see what happens. Thermostat is far cheaper and easier than a converter!

Also I would guarantee that the dealer would miss the engine temp issue.
 

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Discussion Starter #13
It seems I now have a hesitation when cold. What could that be? My mechanic is going to try the spacer on the rear o2 sensor for my 420 code, but this new hesitation means something else is going on.
 

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Cold hesitation could be a lot of things.

Fuel filter
Fuel pump
Vaccum leak(s)
Crankcase air leak
MAF
Not sure if the 2.0 has variable valve timing, might be something with valve timing??

Suggest you get warm idle long term fuel trims and see what the numbers are.

The O2 sensor spacer may help the P0420 code, but this is not really a fix, more of a band aid.

Thermostat being soft could allow more carbon build up and this could be a source of hesitation as well??
 

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Discussion Starter #15
I have checked for vacuum leaks and I don't seem to have any. I know nothing about fuel trim levels. If you could explain I can use my scan tool and check them.
 

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What scan tool do you have?

Can your scan tool display real time engine data like temperature? If so it should be able view fuel trim info if you can read temperature info.

Your tool may have an option like display data rather than check for codes?

I need to know what the long term fuel trim is at idle.
 

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Discussion Starter #17
I checked the long term fuel trim at idle and it was 2.3. I also pulled out the spark plugs and found they were completely worn out. I had a gap of .080. I hope that is my problem with the 420 code.
 

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Discussion Starter #18
I am really starting to get frustrated with this damn 420 code. After replacing the badly worn plugs, I still have the code. The car runs 100% better now and has a lot more power but I just can't get rid of the 420 code. My long term fuel trim at idle is .80

I am going to clean the MAF sensor tomorrow. If that doesn't help I don't know what else to do.
 

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Discussion Starter #20
I can't get this car through an emissions test now and therefore no sticker. a gutted out cat may not help that.
 
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