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Old 02-26-2007, 04:08 PM   #1
Account #: 63635
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Default 16804 - Catalyst System; Bank 1: Efficiency Below Threshold P0420 - 002 - Lower Limit Exceeded

I've asked parts of the following on the B6 A4 forum and on the VAG-COM Yahoo! Groups site with no firm responses. I'm posting here in hopes of having some feedback so I can order the proper parts and tools.

Car is a 2002 Audi A4 Avant 1.8T QMT with 58,100 miles.

No mods, car is bone stock.

Scan fault using Ross-Tech's HEX+CAN for VAG-COM is:

VAG-COM Version: Release 607.3-D
Chassis Type: 8E - Audi A4 B6/B7
Scan: 01 02 03 08 09 0F 11 15 16 17 18 36 37 45 46 55 56 57 65 67
69 75 76 77

VIN Number: WAUVC68E*********
Address 01: Engine
Part No SW: 8E0 909 518 F HW: 8E0 909 018
Component: 1.8L R4/5VT G 0003
Coding: 0016701
Shop #: WSC 63351

1 Fault Found:
16804 - Catalyst System; Bank 1: Efficiency Below Threshold
P0420 - 002 - Lower Limit Exceeded - MIL ON
Readiness: 0000 0000


Question: Am I safe to assume that the fault P0420-002 is related to the secondary, or post-cat, O2 sensor? Or is that a wrong assumption...that it could also relate to the pre-cat O2 sensor? I've STFA, but I did not find any definitive answer.

Question: I want to use VAG-COM to trouble shoot and log measuring blocks related to the O2 sensors and the cat converter. Ross-Tech's site indicates that those groups would be 030-049. Can anyone tell me which groups to log for my car? Or do I log all of the groups? Can someone also tell me what the difference is between:

Bank 1, sensor 1
Bank 1, sensor 2
Bank 2, sensor 1
Bank 2, sensor 2

Thanks for any feedback. Much appreciated.
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Old 02-26-2007, 04:18 PM   #2
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Default Here you go...

<ul><li><a href="http://wiki.ross-tech.com/index.php/16804/P0420/001056">http://wiki.ross-tech.com/index.php/16804/P0420/001056</a</li></ul>
Sebastian @ Ross-Tech.com
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Old 02-26-2007, 04:39 PM   #3
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Default Thanks for the respone Theresias, but

unless I'm a total noob using your Wiki, that info does not tell me anything specific, nor does it tell me anything different than what my Bentley tells me to do. The link in the Wiki links to a thread about guys running HFC's and the correlation of this DTC and people running HFC's. I am stock.

The link you gave was the exact response I received on the VAG-COM Yahoo! Group when I posted a similar question last week.

Are my questions not appropriate for this forum? If yes, that's OK with me. Please don't take my response wrong, I'm just trying to trouble shoot this DTC.

I'm just a guy trying to get more detailed information (if it's possible) to help me spend my money on parts and tools in the most efficient manner while learning more about my car and wrenching on it myself.
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Old 02-26-2007, 05:06 PM   #4
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Default I'm afraid you may be expecting too much.

We don't necessarily have detailed answers or trouble-shooting procedures for every problem or fault code in existence.

Bentley should have detailed procedures for checking the Oxygen sensors on your specific car/engine.

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Old 02-26-2007, 06:09 PM   #5
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Default in about 80% of cases I work on, without any other codes.....

you need a right catalyst. your car, being bone stock, with 58k miles, it should be covered under the federal emissions warranty. My suggestion: bring it to the dealer.
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Old 02-26-2007, 07:32 PM   #6
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Default Thank you Uwe. I appreciate the straight forward response.

I accept that you and your "crew" may not have detailed answers or procedures for everything. I (or anyone else) do not know the extent of your knowledge, and what you feel comfortable in sharing, unless we ask.

There may yet be some other frequent contributors on this forum who can offer some insight. VAG-COM is pretty cool tool to have in my toolbox, and offers a lot. It just seems a waste to me to spend upwards of <a href="http://www.germanautoparts.com/Audi/A4/Fuel/152/6" target="_new">$260+ on O2 sensors</a> if one doesn't have to.
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Old 02-26-2007, 07:45 PM   #7
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Default Thanks for the insight. I'll call my dealer tomorrow and find out if they think they can diagnose

with some degree of certainty that it is the cat or not.

80% is pretty much as close to a sure thing for most top notch techs to say.

At $125/hr, I would hate to end up paying them to diagnose that the cat is fine and its one of the O2 sensors -- that would be the cost for one or both of the sensors to DIY.

At any rate, I will try to figure out if I can diagnose as much as I can with my VAG-COM and multimeter per what the Bentley says. I've already picked up a Bosch Universal O2 sensor for the post-cat sensor that I can return if I don't use it and/or the stock wiring harness checks out to be bad.

In your experience, is there a recommended replacement for O2 sensors? MkTip on the B6 A4 forum referenced that it was 60K. My car runs great, with gas mileage being consistent with the day I bought it.

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Old 02-27-2007, 05:46 AM   #8
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Default more information which may help

First thing I would do is run basic settings and see if readiness can be generated. Second, I strongly advise against installing universal O2 sensors. I've done enough of them and had enough of them come back, that it's not worth the gamble.
Third, here is the bank and sensor info:
Bank 1, sensor 1 = RF
Bank 1, sensor 2 = RR
Bank 2, sensor 1 = LF
Bank 2, sensor 2 = LR
(right and left are as you're sitting in the car)

The complete steps for generating readiness is in the Bentley manual, however, as we're trying to pinpoint a specific area, I'll stick to that area.
Now, by the letter of the book, if we try to generate readiness, meaning force the computer to test a specific component (in this case, the right cat), and it fails, the next step is to replace the cat.

So, here we go. Car is at operating temperature. And I will assume that you're at least somewhat knowledgeable about the use of the Vag-Com.
1) clear codes
2) reset throttle adaptation (04/060)
3) start engine and let it idle
4) 04/030 and check o2 sensor control. you want to see 111 in field 1 and 110 in field 2
5) 04/107, and click start test. wait until field 4 goes from test off to test on to syst. ok
6) leave basic settings, crack throttle open briefly so RPMs go over 3k
7) back into basic settings, enter 034, start test, press foot on brake while flooring throttle. RPM's will increase to about 2300. Test will begin soon after exhaust gas temperature passes 350 degrees. Test will run, and you should see B1S1 OK.
8) now go to basic setting group 036, start test, at idle, and see if the rear sensor tests ok. you should see B1S2 OK.
9) repeat step 8 in setting group 037
10) group 043, start test, brake and throttle together again, and run test until you see B1S2 OK.
11) and now for the litmus test. go to group 046, brake and throttle together, and run the test. test likely won't start until exhaust gas temp goes over 550.

If everything checks out except for step 11 above, rest assured that you have a bad cat. I know it looks like a pain in the ***, but all of these tests together literally take about 5-10 minutes to perform.
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Old 02-27-2007, 11:56 AM   #9
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Default Thanks for the information.

I'll hold off calling the dealer until I can try out what you recommend -- hopefully tonight.

Thanks for the heads up on the universal O2 sensor. I can take it back, spend about $40 more bucks and get a full O2 secondary sensor setup with the harness -- that is if I need to replace it. The extra money is worth it IMO -- at times, I can't afford to be cheap...and being cheap with car parts can, in most cases, end up requiring more money and time in the end.
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Old 02-27-2007, 05:07 PM   #10
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Default Bracketracer, you are awesome!

It was slightly disconcerting to brake and floor the throttle for the first time...expectation is that the RPM's would throw the engine out of the car....but it held RPM's in check as you indicated.

I have some screen shots and data logs from my VAG'ing that I'll update to this thread later, I have to play family man for a few hours this evening first.

Needless to say, your 80% hypothesis was 100% correct, and VAG-COM rocks. I have a Saturday morning appointment with my dealer to have the cat looked at and replaced first thing Saturday morning.

Now I can concentrate on what I think is a bad wheel bearing.....

<img src="http://pictureposter.audiworld.com/63635/step_11_04-046.jpg">
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Old 02-27-2007, 05:07 PM
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