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2002 C5 3.0L Catalytic Converter CEL Question

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Old 03-26-2014, 04:14 PM   #1
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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   #2
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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?
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Old 03-26-2014, 04:30 PM   #3
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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   #4
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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   #5
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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?
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   #6
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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   #7
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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   #8
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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   #9
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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?
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   #10
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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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Old 03-31-2014, 02:53 PM   #11
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Originally Posted by kappa4201 View Post
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
You can't or shouldn't ignore this advice that was mentioned before,
"Your best way to diagnose is to use vagcom to see real time values and pending codes that are not latching but resident."

Those codes can also be from leaking flex pipes and the list goes on so do your detective work and then come back and report the findings.

4Driver4 is spot on with the CTS if it has not been changed, I'd take his advice too.
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Old 03-31-2014, 04:53 PM   #12
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Quote:
Originally Posted by kappa4201 View Post
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
In reading your post, I thought you were content to live with the cat codes but were concerned about poor mileage.

Cat codes are they because the car "needs" cats...according to the crappy programming in the ecu anyway.

Poor fuel economy and cat codes are unrelated.
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Old 03-31-2014, 06:13 PM   #13
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In reading your post, I thought you were content to live with the cat codes but were concerned about poor mileage.

Cat codes are they because the car "needs" cats...according to the crappy programming in the ecu anyway.

Poor fuel economy and cat codes are unrelated.
Are there any symptoms of a 'bad' CTS? Can a partially plugged heater core impact the CTS readings? Some loss in heat, performance, and fuel economy this winter.

Last edited by ajg617; 03-31-2014 at 06:19 PM.
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Old 04-01-2014, 08:56 AM   #14
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Yes sorry to blend the issues but if the cats are ok and won't cause any further damage I am alright with the CEL on. I wasn't sure if poor econ/ foamy oil was a cause/symptom.

I just got in touch with a guy who is going to vagcom my car Thursday.

Is there specific tests that should be done? Sorry I'm not familiar with the software or procedure.
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Old 04-01-2014, 11:52 AM   #15
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in simple terms - the CTS is one of the highest failing parts of our cars , and yet one of the cheapest to replace . i would consider it a maintanance item for every 5 years just to keep it from annoying me.

The CTS will monitor the coolant temp .. it richens the mixture when the car is cold and leans it out as the senses the coolant warm up . depending how it goes bad it can dump fuel in constantly thinking the car is cold. therefore bad fuel consumption and cooked cats.
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Old 04-02-2014, 05:58 AM   #16
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I just want to state for the record, that there is post cat lambda control, and bad cats can in fact cause bad fuel trims.
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Old 04-02-2014, 06:40 AM   #17
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I just want to state for the record, that there is post cat lambda control, and bad cats can in fact cause bad fuel trims.
Here is the fact about your statement right out of this link: http://forums.audiworld.com/showthread.php?t=2580765

Quote>

So, now that we know that the ECU wants to be able to control the A/F ratio. It has a prescribed set of values (maps) for a given RPM, Load, etc. So, the ECU tells the injectors to pulse for exactly XX.X milliseconds and that SHOULD get us the proper A/F ratio that we want. Well, if you tell an employee to go do something, you want to make sure they actually did it, right? The ECU has some snitches (the front O2 sensor and the MAF, for the most part) that will report back whether or not the desired mixture has been attained. The rear O2 sensor is used mostly to monitor the condition of the catalytic convertor, although in some applications it also contributes to trim information.
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Old 04-02-2014, 11:29 AM   #18
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In reading your post, I thought you were content to live with the cat codes but were concerned about poor mileage.

Cat codes are they because the car "needs" cats...according to the crappy programming in the ecu anyway.

Poor fuel economy and cat codes are unrelated.
This is not always true. Converters can melt down, break apart, and be damaged and restrict the exhaust. This will absolutely drag mileage and power down. With the high coil failures seen on 3.0s It doesnt surprise me when I see the fault lights on. All it takes is a few missfires and that cat is super hot, raw fuel hits it and bam overheat meltdown.

We of course dont know if the OPs are that way but removal and inspection would make it clear. It may be like the 2.8's in A6's and the logic to set the code is too sensitive. Set of oxygen sensor spacers seem to trick them pretty good.
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Old 04-02-2014, 02:31 PM   #19
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It appears as though you are disagreeing with me? I am stating, for a fact, that there is post catalyst lambda control on his 3.0.

I can happily show you in the factory calibration if you'd like.
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Old 04-02-2014, 05:14 PM   #20
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It appears as though you are disagreeing with me? I am stating, for a fact, that there is post catalyst lambda control on his 3.0.

I can happily show you in the factory calibration if you'd like.
You should quote me but I get who the post is directed to.

If you are referring to the Bosch LSU 4.7 Broadband Planar technology I don't believe it was used on any C5, we do in fact use the (new design at that time) LSU pre-cat sensor and the LSF post-cat sensor system that go back to the Motronic M5.4.2 that Yes suggest the post sensing does play a role in the "Long Term" fuel trim information not "Short Term Directive" considering that no related DTC's exist.

So here is my question:
Why would any engine management system allow more fuel enrichment
under a known cat performance DTC issue leading to more damage?

I'll sign that class action lawsuit all day long!
 
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Old 04-02-2014, 05:14 PM
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