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Discussion Starter #1
I'm getting codes

P0431 Warm Up Catalyst Bank 1 Efficiency Below Threshold

P0421 Warm Up Catalyst Bank 2 Efficiency Below Threshold

where do start for diagnosing? It looks like both driver and passenger side, but, do these codes relate to Upstream (before the Catalytic Converter?) or Downstream (after the cat)?

Is there a way to test the O2 sensors, because if the sensor is bad, I will replace it, if the sensor is good, I will add a spacer. Would O2 sensors on both sides go bad at the same time?
 

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Is there a way to test the O2 sensors
Do you have a scanner to read the live O2 sensor values?

Would O2 sensors on both sides go bad at the same time?
Very unlikely that both go bad at the same time. Nevertheless, that is not the issue here. Just get the values and see if they indeed gone bad.
 

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These are for the downstream sensors. It could be bad flex pipes as that allows air into the exhaust.

If the flex pipes are good, just add spacers to the rear O2 sensors. On earlier V6 engines, there was a software update as the parameters were (supposedly) set too low.
 

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age/mileage?
o2 sensors ever been replaced?
cats ever been replaced?

I thought you got the 2096 and whatever the other side error is when you have flex pipe hole issues...4031/4021 usually was 02 sensors.
 

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Discussion Starter #5
I don't have have access to the car right now, but as soon as I do, I will check things and reply back. I just happen to have some spacers so I might start with adding spacers and then check the VCDS Vag-Com software again. The flex pipes are a bit worn but still solid and seem to be sealed tight.
 

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Leaks most likely at the flex joints. You can start it cold and run your hand around joints, you should feel something before it gets too warm to touch.
 

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Discussion Starter #7
Thanks, Btitans, I appreciate the tips about the flex pipes. I wanted to know how they could be checked for leaks.

Thanks, Emry, PZ and ar393, I didn't realize how much could be checked with the VCDS (VAG-COM) software, I generally just use it to look for fault codes.

I turns out that the Fault codes were caused be one of the cylinders misfiring. I completely forgot that I couple of months ago, this same car (my wife's car), had the #2 cylinder spark plug wire go bad. I checked the software and just like last time, one of the cylinders was misfiring a lot. I replaced that plug wire (the wire that runs from the coil pack at the front of the V6 engine, to the spark plug), and the car has been running great since then, no mis-fires.

I plan to buy a set of high-quality plug wires and replace all the wires.

Thanks again for your help and advice.
 

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I turns out that the Fault codes were caused be one of the cylinders misfiring. I completely forgot that I couple of months ago, this same car (my wife's car), had the #2 cylinder spark plug wire go bad. I checked the software and just like last time, one of the cylinders was misfiring a lot. I replaced that plug wire (the wire that runs from the coil pack at the front of the V6 engine, to the spark plug), and the car has been running great since then, no mis-fires.

I plan to buy a set of high-quality plug wires and replace all the wires.
Hehe, I need replace my #2 Cylinder wire now myself. At some point down the line I may get some higher end wires.
 

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have to chuckle......it's like my car is reading/listening to these posts, I'll read something about .............vacuum leaks and misfires as an example..... and on the way home I get misfires/vacuum leaks......,however glad you got 'er solved
 
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