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Discussion Starter #1
I've got a 2000 AEG/5sp that's been running a bit rich. It pops a CEL about once every tank (400miles). Usually the CEL is "Bank 1 too rich" or simular, however, today I got the "Catalytic Converter" code.

I hooked up the VAG-COM and went to the O2 measurement blocks. Sensor 1 seems to be stable around 200mV with a 8.6% additave lambda.

However, Sensor 2 hovers around 700mV.

After letting the car idle 20 minutes as I was playing around, I noticed both sensors jumping between 150mV and 800mV.

Any thoughts anybody?

Thanks!
 

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Wife's 01 1.8T mechanic
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The readouts should be quickly raising and lowering. The sensor before the cat regulates the A/F mix, the one after simply lets the computer know that the cat is working.
 

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Thanks for the reply...

Yup that's what it looks like, but while idling too?

I figured they'd be pretty steady while the car was just sitting there at 760rpm.

Any 'cross checks' with other measuring blocks I can look at?

How does a VW mechanic determine bad 02 sensors VS a bad catalytic converter?
 

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The readouts should be quickly raising and lowering. The sensor before the cat regulates the A/F mix, the one after simply lets the computer know that the cat is working.
Well a couple of things here....

First your codes and descriptions don't seem to coincide, you say that you are getting a code for Rich Fuel Trim and have an 8.6% additive trim, this means the car thinks its lean and is trying to add fuel un less its a negative number? What is the multiplicative trim number?
When you clear the fault codes it will also reset your fuel trims, you can clear and go for a drive and see what numbers return.

Second the O2 descriptions posted above is not entirely correct, the front O2 description is correct, the rear is not.
The rear O2 sensor should have a more consistent readout, with an average voltage of .455v.
If the rear is rising and falling along with the front then the cat is not doing its job...



Here is some links for you...
Fuel Trim Info - Ross-Tech Wiki

Oxygen sensor

Hope this helps...
 

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Well a couple of things here....

First your codes and descriptions don't seem to coincide, you say that you are getting a code for Rich Fuel Trim and have an 8.6% additive trim, this means the car thinks its lean and is trying to add fuel un less its a negative number? What is the multiplicative trim number?
When you clear the fault codes it will also reset your fuel trims, you can clear and go for a drive and see what numbers return.

Second the O2 descriptions posted above is not entirely correct, the front O2 description is correct, the rear is not.
The rear O2 sensor should have a more consistent readout, with an average voltage of .455v.
If the rear is rising and falling along with the front then the cat is not doing its job...



Here is some links for you...
Fuel Trim Info - Ross-Tech Wiki

Oxygen sensor

Hope this helps...
Yep, the rear should be more steady since it is for checking cat operation. Also it is normal for it to take a while to start operating since the cat has to warm up before it is an accurate reading.

However you need to find out what is causing the (possible) cat failure since you had the too rich code prior to the cat code.

A bad post cat sensor would likely throw a fault for that sensor if it was a voltage issue.


Sent from my iPhone using AutoGuide.com App
 

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Discussion Starter #6
Much more data...

I went for a long 'spirited' drive up and down Houston's freeways and the CEL came back on. Goody! When I got home, I whipped out the VAG-COM and started recording. All these readings are with the car warm and idling:


Control Module Part Number: 06A 906 018 JQ
Component and/or Version: 2.0l R4/2V MOTR HS V01
Software Coding: 00031
Work Shop Code: WSC 00037

1 Fault Found:
16806 - Main Catalyst: Bank 1: Efficiency Below Threshold
P0422 - 35-10 - - - Intermittent


group 30 - 111 111

group 31 - sensor 1 = .085v to .850v sensor 2 = .165v to .750v
-both sensors trending fairly equally, post-cat sensor voltage following the pre-cat.


group 32 - -8.6%, 0.0% (running rich, no multiplicative lambda)

group 36 - fluctuating from .140v to .725v, B1-S2 OK (does this mean the post cat sensor is good)

group 41 - 29.3 ohms, Htg. S1 ON, 26.8ohms, Htg. S2 ON

group 46 - 760 RMP, 368C, 0.0, Test OFF (idle RMP, cat temp, etc)


I'm also attaching a picture showing both sensor voltage levels over time. The red line is Sensor 1 (pre-cat) the green line is Sensor 2 (post-cat). It's fairly easy to see that as the precat sensor peaks to .700v+ the post cat sensor peaks shortly there after.

Opinions?

Should I replace the post cat sensor?

Does this confirm my converter is bad?

Do I need more data to make a determination?
 

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I would check and see what is not passing the readiness code. If the 02 sensors are passing and the cat is not than I would guess cat. A dealer should be able to tell you for sure in an hour diagnosis time (1 hour labor).

If it was my car I would think catalytic converter, especially since the rear sensor was fluctuating so much, that should not happen.


Sent from my iPhone using AutoGuide.com App
 
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