yagovoz

I have a 2001 Audi A6 quattro avant. I was removing my exhaust to do some driveshaft work and was a little confused by the "post-cat" o2 sensors I had to remove since they are not actually after the cats, but just before them. My Bentley shows the diagram with the o2s just downstream of the cats. Is it possible someone welded my cats in backwards in the past? (just so you know, I'm not confusing these with the pre-cats near the manifold.)

In any case it doesn't seem the so-called "post-cat" o2s would serve their purpose where they are now (upstream of cats). And it is interesting to note that I've had a "cat converter below peak efficiency" CEL code, although I thought it was from the massive exhaust leaks at the flex pipes that I'm about to replace.
I need to know so I can rectify it if necessary while the exhaust is all dismantled and what not.
Thanks.

RMsALFA

'post cat' sensors should be just that, after the cats. They don't do much except monitor the 'health' of the cat <shrug>

yagovoz

I know that the post-cat o2 sensors do nothing more than check the efficiency of the cats. I'm just a little confused about why they would be located where they are. Maybe someone that stupid DID get a hold of this car at some point and put the cats in backward.

SloopJohnB@mac.com

Anything is possible. You could check a Bentley for component location or a dealer parts fiche (yuk yuk, betraying age...it's all on disk or online now) for location although they're not always accurately pictured.

You could even post a picture on this forum....

GTA_Driver

Do you mean that both O2 sensors are before cat #1 ?

As in, an O2 sensor, another O2 sensor, then a cat, and another cat?

rs4-380

it's a stock setup. What you are seeing underneath the car is actually the 2nd cat (on each side). The initial cat is upstream in the downpipe, the 02 sensor threads into the secondary cat on the inlet side.

yagovoz

I was beginning to suspect that. My bentley diagram shows the o2 sensors for my model and engine on the downstream side of the cat, but I'm assuming that Audi made some of the C5's with no "pre-cats" thus making it necessary to have them there. My car apparently differs from what my much-abridged repair manual shows. Thank you all for clarifying.