I recently bought a 2000 A6 Avant and I am trying to get it through inspection. The check engine light is on with codes P0160 and P1177. P0160 is O2 sensor B2 S2 - no activity. I bought a new O2 sensor, but when I went to replace it, I saw my S2 sensors are located in front of my catalytic converters. Everything I read about Audis shows these sensors located behind the cats. Is that true and if it is, could my catalytic converters be installed backwards? I attached a picture of the S2 sensors.

 

You have 4 O2 sensors, B1S1, B1S2, B2S1, B2S2

B=Bank
S=Sensor
B1=passenger side
B2=drivers side
S1=pre-cat
S2=Post cat

So your B2S2 error is your drivers side post(after) cat sensor.

So the 02 sensor you found is your B2S1 sensor.

 

I know there are 4 sensors. The sensors in the picture are the S2 sensors. I have no sensors behind the cats.

 

Here is another picture from the front looking back.

 

Based on the picture, someone put on aftermarket converters and did a bad job of it.

First check the O2 sensor wiring connectors. There are 3 on the passenger side behind the air intake hose. (Green, beige, and black connectors below), and one black on on the driver side. (No it is not my engine, and yes it is ugly)



Black ones are for the front sensors and the green and beige for the rear ones. Trace the wiring and see where they go. Hopefully they are still there.

 

Does that mean catalytic converters can be installed backwards? Can that damage anything if they are?

 

Who knows what they did and which one they installed... I can't tell.

 

I'm not positive on this but I think your sensors are placed correctly. They are after the cats because of the cat that is just past the exhaust manifold. Why audi didn't put them farther back is beyond me. Maybe the second one is just for good measure. When I hit the tester I was WAY!!! under the limits for passing emission. someone correct me if I am wrong

 

According to the Bentley repair manual I have, the cats are under the car 1 to 2 feet from the manifolds. The S1 sensors are right off the manifolds and the S2 sensors are right behind the cats. Like I said, it looks like the cats are installed backwards. Can anyone confirm this from the pictures and what problems might this cause?

 

Sorry, I stand to be corrected. The location of the sensor is correct. There is an upstream component and your sensor one is before that. The sensor 2 location is correct. I had to crawl under the car to check. Sorry for the confusion.