2002 Audi A4 O2 Sensors

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

6 Replies

2CARPRO JACK

MECHANIC
11,533 POSTS

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

SPONSORED LINKS

Was this

answer

helpful?

Yes

Monday, May 18th, 2009 AT 8:16 AM

BMGBMG

MEMBER
4 POSTS

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!

Was this

answer

helpful?

Yes

Monday, May 18th, 2009 AT 8:42 AM

2CARPRO JACK

MECHANIC
11,533 POSTS

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?

Was this

answer

helpful?

Yes

Monday, May 18th, 2009 AT 11:41 AM

BMGBMG

MEMBER
4 POSTS

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!

Was this

answer

helpful?

Yes

Monday, May 18th, 2009 AT 12:15 PM

2CARPRO JACK

MECHANIC
11,533 POSTS

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