Just ordered new downpipes cat and muffler for my 97 a4 2.8. I wanted to re-use my 02 sensors in the new piping, unfortunately, the rear o2 sensors would not come out while the pipes where attached to the car, so I cut the wires of the rear o2 sensors, and later removed them in a vice. Problem now, I thought there were only 3 wiresper o2 sensor, but the rear o2 sensors have 4, 1 black 1 grey and 2 white. I need some help in order to re-connect the wires back together, does anyone have a link or diagram explaining how an o2 sensor works and is wired into this car? Also, if anyone knows where the rear o2 sensors plug in up front, I could trace the wires back to the plug and try to make some sense of it there, but I dont know which plug is for the rear o2, THANKS IN ADVANCE,

 

All the O2 sensor connectors are on the firewall right there on each side of the engine. Two on each side.
Don't bother re-wiring them with connectors, you will end up getting fault codes from resistance in the circuit.
Just get both the sensors from www.ecstuning.com, they are a lot cheaper than the dealer.

 

Your talking 0-1v on the signal wires. Not really room to spare on a bad connection.