2003 3.0. Did a pretty complete search but failed at finding any detailed info on replacing the passenger side pre-cat O2 sensor. Plenty of comments and pictures indicating it as a true PITA but found nothing explaining how a person actually went about changing the thing. Any help or sources appreciated.

 

On out 2.8 it took about 25 mins, we just moved a bunch of **** out of the way. We worked from above, we removed the intake tube and air filter box. I think that was it. I'm pretty sure it should be about the same on a 3.0 as it's just the big sister to the 2.8

 

I had found a 2.8 write up but it appears there is a good deal of difference between that engine exhaust and the 3.0 (Audi obviously discovered the O2 sensor was too easy to replace and fixed that problem on the later model). Directly behind the head on the 3.0, there's direct access to the post cat sensor (I actually pulled that one believing it was the pre cat...silly me). Reaching down behind the engine and several inches in front of the post cat, I found the pre cat....it actually leans into towards the motor. I can touch the thing...can't see it...questioning a way to actually even manipulate a tool.

 

Obviously you probably got the right idea, even though I've never seen a 3.0.
On a 2.8 C4 (at least on a 1995) is as simple as removing the air duct in between the MAF and Intake box, disconnecting the sensor, removing the sensor, putting the new one in, connecting it, reinstall the air duct, button it up.

 

Your picture of the 2.8 pre cat O2 is a near perfect example of where the 3.0 post cat O2 is.

 

Shamrock, as I said, I know nothing about the 3.0 - as I'm yet to see one.
However, a "post-cat" cannot be located before the cat itself - it would defeat its purpose or title/characteristic quality - if I may. Do you agree?
If on the 3.0 the "post-cat" sensors are placed in the same place like the pre-cat ones in the 2.8, then what do you have on top of (or after) the cats?
Do you have any sensor whatsover in the cat area?

 

I'm also not familiar with the 2.8 other than in the most basic sense. I have a Bentley service manual that includes the 2.8 as well as the 3.0. From what I can tell as far as the book shows, the 2.8 cats are much further downstream from the exhaust manifolds than the 3.0. The 3.0 actually shows the cats (smaller sized than the 2.8) bolting directly up to the exhaust...manifold flange to cat flange...no exhaust piping inbetween. Found this picture of a 3.0 motor removal and it shows the passenger side cat and O2 sensors....they appear to be literally within 6 or 7 inches of each other. Looks like fun...

 

Oh, I see now - thanks for the pic.
It is indeed a totally different design, without any downpipes. Due to the angle I picture, it seems that those pre-cat sensors are indeed very challenging to get to. You definitely need one of those lambda wrenches. Good luck.

 

I think I exhausted all my luck when I changed the neutral safety switch a couple months ago. After that one, figured I had faced the most frustrating repair imaginable....everything would be easy after that one. Oh well....good excuse to go buy one of those inspection camera's...if I explain the purchase and need properly, wife is sure to be understanding.

 

Was it back-up lights or shifting related?
I have no back-up lights ever since I bought the car and after reading I figured I need to replace my neutral safety switch as well, but never got around doing it.
Planning on pulling the engine out in the spring to replace the pan upper gasket and the rear main seal. That would be a good opportunity to look at the TQ, replace all the differential seals (especially the two inner ones - "drive axle seals" are called) with the halfshafts off, and replace the neutral safety switch... I also need control arms and maybe new TREnds...
It is an expensive undertaking, since I'd need an engine hoist and a tranny jack, but I hope wife will not be too upset about it, either.