Audi Repair: Oxygen sensor replacement, o2 sensors, catalytic converters

Question
Jan,

My 01 Allroad's CEL went off.  The code was p0135.  I read changing the O2 sensor should fix the problem... the problem is:
1) What's the best way to get at those hard to reach O2 sensors?
2) Where are the wires going?  I need a good place to splice into with my Bosch universal replacement.
3) Should I replace both front O2 sensors?


Thanks in advance.

Answer
You will have three or four wire O2 sensors.  Make sure the Bosch universal replacement is compatible; I have heard of a few sensors recently that were DOA.  The wires go to a plug located typically just behind the engine.  Your engine has 4 O2 sensors; two in the 'hot' stage just before the catalytic converters, and two in the back of the catalytic converters.  The job is best done with the right tools and small hands; no shortcuts I'm afraid.  The ones in the cats are easiest done from underneath, the front ones you can see in the downpipes.  If I remember correctly, the one on the passenger side is much easier than the one on the driver's side.  I replaced both front O2 sensors on my '01 S4 at once; you don't have to but they both 'enjoy' similar environments so they typically fail at similar times.  Let me know how you get on.  Jan