2002 Audi A4 O2 Sensors

Tiny
BMGBMG
  • MEMBER
  • 2002 AUDI A4
  • 6 CYL
  • AWD
  • AUTOMATIC
  • 76,000 MILES
The SES light came on, Code PO431, I wanted to replace the o2 Sensors, I was told there were a total of 4 o2 sensors, two in front of the cats and two after the cats, where are the 2 rear or after the Cat sensors located? I can barely see 2 sensors (1 each side) that appear to be the front sensors, they are right in front of the Cats and just behind the manifold, where are the rear sensors located? Where can I find a diagram showing this location and instructions on removal?

FYI I already purchased @ $16 your e-auto repair manual and for the life of me can not find and info to locate and replace these.

Thanks

Brian
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Sunday, May 17th, 2009 AT 5:58 PM

6 Replies

Tiny
JACK42
  • MEMBER
That code is for cat ineffeciency. You can try replacing the rear most sensors, but it is usually the cat converter that is the problem.I too couldnt find any locations for them other than "In exhaust system" Keep looking, there should be 2 behind the cats, they may be back a little bit due to the heat created by the cat. Be sure to use OEM sensors
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Monday, May 18th, 2009 AT 8:16 AM
Tiny
BMGBMG
  • MEMBER
Thanks for responding so quickly. I had cleared the code with my scan tool and it has not come back. Yet but the car is running slugish. From my research the sensors are usually the culprit and a heck of a lot cheaper to replace. Either way the Cat or the sensors will need to be replaced soon enough so I started with the sensors but. Where are they? There are 2 small cats directly off the maniold, then there is a larger Cat mid way down and then the mufflers, it that larget unit even a cat? I have searched high and low and cannot find the "rear" O2's to save my life. If you do not know the exact location how can I find out where they live?

Thanks!
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Monday, May 18th, 2009 AT 8:42 AM
Tiny
JACK42
  • MEMBER
Unlikely the big one is a cat, otherwise the PCM couldnt distinguish between bank1 or bank2. If the performance is alittle sluggish, you may want to look at the cat as the culprit, when they get partially plugged they will affect performance. Have you tried Google for a club or blog on the matter?
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Monday, May 18th, 2009 AT 11:41 AM
Tiny
BMGBMG
  • MEMBER
Hmmmmm. Man I have been all over the web looking for help, a diagram, pictures, anything. And I purchased your manual and found nothing in there!
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Monday, May 18th, 2009 AT 12:15 PM
Tiny
JACK42
  • MEMBER
I hear ya, I have been looking all over too. Are you sure the ones you can see are the primary sensors? Look closer to the engine to see if just maybe there is another set upstream. If not there has to be something downstream, look under all shields too
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Monday, May 18th, 2009 AT 1:10 PM
Tiny
BMGBMG
  • MEMBER
Well they sure look like they are inbetween the manifold and the Cats. I can barely get my index finger on the O2 sensor on one side. The other side I can't touch. If it turns out that these are the rear O2's I need to know how to get to them, even if I decided to change the cats and the O2's I need to know how and that Manual I signed up for doesnt show how to take anything out. Seems like the manual is not much good to me. Any suggestions?

Thanks
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Monday, May 18th, 2009 AT 1:42 PM

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