My 02 S4 runs good one day and not so much the next expecially when its cold. I scanned my codes and found I got a bad O2 sensor. My mechanic is telling me its my last sensor down in the exhaust. I tried to remove them but cant because it just twists the wires. I know im suppose to disconnect them before I take them off but I followed them up and they seem to go on forever. Any Idea where these sensor wires connect to? Wish I had the number of the code that came up but I lost it! ugh thanks!

 

Sensor wires are connected to the control module.

 

In the engine area? On top?

 

It is on the rearmost section of the engine area on the driver's side.