My 2001 A6 avant threw a "P0431" code. I tookit to the dealersince the cat is still under the warranty. Anyways, they indicated that they will cover the catalytic converter replacement under thewarranty.Also they recommend that my upper o2 sensors to be replaced since they are reading a bit low, and the sensors will not be covered under the warranty.Their quote, plus minus $650...

So here are my questions.

1. Should I even worry about replacing them since there was no code thrown for the o2 sensors?
2. I noticed that I can get a Bosch universal o2 sensors for $75 or so each, not $250 as dealer quoted. Will it work?
3. Are the upper sensors easy to access if I want to replace them myself?

Any inputs greatly appreciated.

Thanks.

BTW it is 2.8 30V V6.

 

i would just have them replace your catalytic converte, but just forget about the o2 sensors right now, if a dealership says there low then you probably have a while before you actually have to replace them, but if a good mechanic friend says there low i would replace them asap.

 

Well... my opinion (and it's worth exactly what you paid for it...

O2 sensors don't just read "low." They swing or transition from a low voltage to a high voltage (~.1V to .7 or .8V on average) as the mixture goes from lean to rich several times per second. In practice, this voltage is constantly changing and the transition from high to low (across a .45V threshold) is called a cross count. As O2 sensors age, they generally become "lazy" and produce fewer cross counts per given period of time. A damaged O2 sensor is a different story, but the idea that yours are "a bit low" makes no sense to me. If they told you the "cross counts" are a bit low, then that would make more sense, but mine had 140k on them when the PS one finally started throwing an occasional code for the heater, but the cross counts were still good, and indeed, I noticed no improvement in mileage or performance after replacing them. I would certainly ask for more detailed information on why they believe the sensors should be replaced and see what they say... most modern cars will begin throwing codes when the cross counts fall below a certain threshold to let you know the O2 sensor is getting tired (this may or may not turn on the CEL, but will show up when you scan for codes). If the sensor has truly and completely failed, you will definitely throw a code and light the CEL, so IMHO, you don't have enough information to make a decision. If they can't provide better support for their diagnosis, then I would be wary of just dropping the $$ on their mere suggestion. I would want a second opinion.

You can get Bosch sensors with the Audi connector for around $150 each and the generic tool is about $10. On a 2.8L, replacing them both is a one banana, one beer job. I have no experience with the other motors.

HTH

JJJ
1999 A6 Avant 2.8L

 

Thanks for the info JJ. I agree with you, the dealer really hasn't given me enough information. As for the code readout, I asked to see them but they would not share them with me. WhenI use my generic OBD II reader, I only get a code for the catalytic converter. Unless they have an Audi specific code that my reader is not reading, I really doubt that I need new o2 sensors.

As for the upper o2 replacement, can I access them from below? Because I was not even able to see them from my quick glance from the top. Do I need to remove some things to get to them? I probably wouldn't replace them but would like to know for my future reference.

Let me know. Thanks.

 

You get to both from the top... remove air intake duct on PS and move coolant reservoir aside on DS if I remember correctly. The PS is a little hard to get a hand on... thus the need for a crow's foot O2 socket such as:

http://www.bosstoolsupply.com/index....p;ProdID=11814

but they really aren't that hard to do. At 140k miles, mine were not "frozen" in place and a little persuasion with a good ratchet was all it took to loosen them. You can get that socket at any AutoZone and probably other chains as well.

I don't think we've talked about mileage and such, but the way the computer detects a bad CAT is by comparing the manifold O2 sensor readings to the O2 sensor after the CAT. It is possible, I suppose, for a bad O2 sensor to cause the ECU to throw a code for a bad CAT, when in fact, it is only a bad O2 sensor.If they replace the CAT and you still throw the CAT code, then an O2 sensor was likely the culprit to begin with. This is starting to sound a little like they want to replace everything that could possibly be related to that code just to insure you don't have any further problems... fine if it was all under warranty, but since you have to pony up some cash, I'd want them to make an actual diagnosis rather than "shotgun" parts.

How many miles on this car?

JJJ

 

P0431 = cat

 

Thanks for the great info. The car only has 79,000 miles. So the cat is still under the federal warranty. Interesting thing is that I had a chance to look at the previous service records from audi a few weeks back. A cat was already replaced at 40,000 miles or so. But of course I don't remember if it was the same one or not.

I scheduled to replace the cat on next Wednesday after the new year. I will let you know whatever comes of it after the replacement. Thanks for the info, once again.

 

Replace the cat either way. Putting a new one in there will not hurt anything and its free so let them do it. As for replacing the o2 sensors...if you don't get a code for it don't bother. Or as you said, if the current code persists after the cat replacement then replace them. They are difficult to get out but not because of techical difficulty. Just that the constant changes in heat seats them in there really well. If you end up replacing with a generic (not a big deal) then you will have the splice the connector off of the old one onto the wires of the new one. Also check that you get the same kind (2 or 3 wire).

 

Update: The bank 2 pre cat was replaced under the federal emmission warranty and the CEL went away. Rediness code reset after the installation and all is good. I guess the dealer wanted to make extra new year cash by offering O2 sensor replacement; but, you know they are ensuring their survival and who can blame them... Thanks for all your inputs.

 

As a mechanic I really do not agree with that attitude. A true mechanic understands the problem and fixes that problem rather than simply replacing all parts related. This is part of the reason all dealers seem to get bad reputations. They are taught (not all of coarse) that when in doubt, replace.

In your case for example, the possibilities were with the cat or one of the two o2 sensors. Them recommending you to replace those parts at your own cost is wrong. The correct procedure would have been to fully inform you of the possibility of them being worn. You as a customer should be able to trust the mechanic to recommend what is necessary only. Not to simply replace till it works.