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2002 C5 3.0L Catalytic Converter CEL Question

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Old 03-26-2014, 04:14 PM
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Default 2002 C5 3.0L Catalytic Converter CEL Question

Hello Everyone.
First post but have been reading the forums for a while. I can't seem to get a straight answer from the dealer or shop so hope someone can help.
Bought a 2002 A6 Quattro 3.0L(AVK) a few years back with about 150K miles on it. After driving it a few days CEL came on, threw two codes: P0421 and P0431. After doing a google search most posts pointed to catalytic converter issues so I called the dealer and they said the parameters are set too tight in that vehicle but everything is fine and if I wanted the light out it would be $2000 a piece to replace the cats. Not good news to hear.

So I put it out of my mind and loved it all summer (never drove/owned an Audi before) until Minnesota winter, then the quattro really made me an Audi convert. All was good until the short commutes (<5mi) made the oil very foamy the color and consistency of butterscotch pudding. (No loss in coolant reservoir). Again the dealer said this is normal with these cars and short trips but I never found a picture with as much foam as I had under the oil filler cap. Eventually froze the PCV and blew the passenger camshaft adjuster seal. Fixed that then heated the car once a week in a shed then took extended drives to try to burn off moisture between oil changes. The next summer my milage kept decreasing and the car has been losing power since. This was the second winter and no oil leaks but I've been very careful to drive at least 10 miles to and from work as well as extended trips. Still getting a lot of foam but I keep an eye on it. The oil smells very gassy as well.

Sorry for the long backstory but my question is:
If the cats are truly bad, would the codes thrown make the car run rich all the time? Would this lead to poor milage and combined with short trips in cold weather overtime cause unburnt fuel to move down piston walls and cause the oil to get very foamy?

I read that both codes constantly on at the same time is almost always bad cats and not O2 sensors. Is this true?

Hope someone can help me out.
Thank you!
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Old 03-26-2014, 04:24 PM
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(Quote)If the cats are truly bad, would the codes thrown make the car run rich all the time? Would this lead to poor milage and combined with short trips in cold weather overtime cause unburnt fuel to move down piston walls and cause the oil to get very foamy?

First answer is No, the cat converts unburnt hydrocarbons, the pre O2 sensors adjust fuel trim and the post O2 sensors report the condition of the cats.

Second answer is No, the "tan butter" is moisture in the oil in the crankcase vent system none of this oil would be in the level of the oil pick-up in the pan.

What codes are you getting now?

Last edited by jcman; 03-26-2014 at 04:40 PM.
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Old 03-26-2014, 04:30 PM
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Default Just to add

If the cats are restricted then mileage and power will suffer.
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Old 03-26-2014, 07:32 PM
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Since I got the car they have been P0421 and P0431. Warm Up Catalyst Efficiency Below Threshold (Bank 1)/ (Bank 2). Clearing the codes removes the CEL for about a day until they come back on.

My thought was the system sees the cats are not heating up properly and dumps more fuel to raise their temp. But by adding more all the time a lot goes unburnt, making the situation worse.

And moisture in the case should get burnt off as the car reaches temp. But the foam comes back quickly and smells very gassy so that is why I believe the foam is from fuel in the oil versus water.

Also I have not done a pressure test at the pre-cat 02 to measure back pressure to indicate restricted flow. I can't find a gauge to rent in my area, I may just have to buy one.

Can anyone explain to me how the car computes the P0421/P0431 code?
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Old 03-27-2014, 05:09 AM
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Originally Posted by kappa4201 View Post
Since I got the car they have been P0421 and P0431. Warm Up Catalyst Efficiency Below Threshold (Bank 1)/ (Bank 2). Clearing the codes removes the CEL for about a day until they come back on.

My thought was the system sees the cats are not heating up properly and dumps more fuel to raise their temp. But by adding more all the time a lot goes unburnt, making the situation worse.

And moisture in the case should get burnt off as the car reaches temp. But the foam comes back quickly and smells very gassy so that is why I believe the foam is from fuel in the oil versus water.

Also I have not done a pressure test at the pre-cat 02 to measure back pressure to indicate restricted flow. I can't find a gauge to rent in my area, I may just have to buy one.

Can anyone explain to me how the car computes the P0421/P0431 code?
Sure I can explain how the codes are determined, the post O2 sensor signal value behind the cat is compared to the pre O2 sensor so in detail if the post signal is the same the cat is not efficient.

In the exhaust system the only component that regulates fuel trim is the pre O2 sensor before the cats the CTS is the main one in the engine, gas in oil does not foam.

Your best way to diagnose is to use vagcom to see real time values and pending codes that are not latching but resident.
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Old 03-27-2014, 05:17 PM
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Thank you for the explanation.

When you say "cat is not efficient" would you assume they are restricted/plugged or just not doing their job?

What do you think the logical next steps are? Check back pressure then vagcom for 02 sensor function?

Worst case I will just replace both sides. I think I can do it for under $1000 but I would rather have it in my pocket. My goal is to get about 4 more years from this car.

Thanks
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Old 03-28-2014, 02:59 AM
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There is nothing wrong with the cats. Don't waste your money. Look elsewhere for the mileage issue. Coolant temp sensor new?
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Old 03-31-2014, 10:39 AM
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I've never touched the coolant temp sensor. I'll look through the service log but I don't think it has been replaced. Why do you ask?
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Old 03-31-2014, 11:50 AM
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Originally Posted by kappa4201 View Post
I've never touched the coolant temp sensor. I'll look through the service log but I don't think it has been replaced. Why do you ask?
If you have to ask , you havent been reading these forums as long as you say you have
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Old 03-31-2014, 01:45 PM
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Let me revise "reading forums for awhile" then lol.

From the posts I read about P0421 and P0431, I never saw any remarks regarding the coolant temp sensor.

Can you help clarify the coolant temp sensors role in this problem?

Thanks
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