fyreous

Hey all,

These were some great instructions! Followed them last Saturday and was able to get the pre-cat O2 sensors changed out in my friend's 2001 A6 in about 2.5 hours. HOWEVER, I want to make a brief comment about the post-cat O2 sensors...

They are definitely easier to reach, as the instructions said... just lift the car up onto some jack stands and slide under. You can see them right there. The problem is the routing of the wires on the underside of the car up into where they plug in under the hood. They are routed above the driveshaft. Unless I am missing something, there is NO WAY you can re-route the wires for the new sensors in the same place as the old ones without some major disassembly. The space seems too small to fit any tools or hands up into, and getting zip ties back on looks impossible.

Anyone have any direct experience changing the post-cat sensors on this car? Am I missing something?

Thanks!

Airbag

Thats funny .. as an add on to that last post that is exactly what i saw .. jacked up the car after my wife got home expecting it to be a fairly quick job. realized the wiring for the post cat sensors dissapears into nowear. looked down the top and realized there is a fair bit of dissasembly up there to even get to the tie wraps I can see.. Im sure it can be done but will need saturday to do .. niot just a quick job before it gets dark thats for sure.

Airbag

well i started the post cat o2 sensors and what a feakin nightmare .. to be honest I could have just cut the wires and ripped the harness out but i didnt.. asi dont know where to splice in my new one. of course the throtle body at the back has to be removed and then you have to loosen the rubber on the hose of the pcv valve thing just so you can get your hex driver in to take the throtle body off .. this .. perfect example of poor design without input.

the reason you take this off is to get to all the tie wraps uder there holding the passenger sensor that plugs in on the passenger side .. yup they run the cable under there coz they had extra ... after getting those off you realize they ran the O2 sensor cable between two transmision coolant pipes and the transmision .. just because they could, then go up to the engine bay ,,, could they have gone over top of those pipes ? sure .. but that would have made replacement easy .. so you are feeing along down there with your arm jammed for a spot where you can either get the connector through or the o2 sensor through between the steel pipes and the transmission . nope ..

i eventually found a spot i could barely touch on the far passenger side where the pipe moves away from the transmision .. i am thinking the connector might go through because its slightly narrower than the O2 sensor .. nope it jams and i can barely get my fingers on it now .. i decide to try and force it through as i have no choice. got it through but broke one of the clips..

great ...
then i look down way under and i see another fastener fastening the o2 sensor harness to the transmision .. frustrated i just pull it and it pops off ..

and i haven't even gotten to the under side yet .. i gave up in frustaration and i will start underneath tomorrow ...

am i doing this all wrong or have other shared this frustration of it being way harder than it needed to be ... ?

interesting note though .. car has 350,000km and by the looks of things these are the first o2 sensors to go in, as all the tie wraps look original, and there is no way they would put it back under that pipe.

twinblus4

I have a rich code, oxygen sensor (at work now so I don't remember the verbage)

I have vag-com, is there a way to find out exactly which sensor needs replaced?

Esquire6

When I replaced my pre- and post-cat O2 sensors with the Bosch "universal" version, I found that the passenger pre-cat sensor was the one that gave me the most trouble.

For the post-cat sensors, I made the wire spice fairly long rather than risking it being too short.

While underneath the car, (I had access to a lift) I removed the sensor from the bung. Then I tied a piece of fishing line to the end of the sensor and anchored the other end of the line to the exhaust pipe. Afterwards, I unplugged the sensor from its position on the firewall in the engine bay. Then from the engine bay, I pulled the sensor wire up through the maze beneath.

From underneath the car, I pulled the newly spliced wire down through the maze using the fishing line. Of course both wires snagged on everything that was possibly "snagable" each way but they eventually made it through.

vtraudt

Is this the right 'universal' Bosch?
Bosch 15733 Oxygen Sensor

dwengi

I did the post cat sensors about a year ago. The wires appear to be secured to the top of the trans; after a couple hours I could not figure out how to remove and gave up. I just ran new wires and zip tied them where I could.
Would be interesting to find out the factory procedure for this service: step 1 - remove transmission??

Ceedawg

Guess I will gladly pay the 250.00 to have some one change all four of these babies. Wow!!

Ryan Waldron

Would someone be kind enough to re-host the photos above? Thanks much to the source

desertsage

Hi all! I s this thread still active? I am thinking of taking on replacing all 4 of my old O2 sensors. I crawled under my B5 (2.8L 12v) yesterday and, like most of you, was struck by the apparent difficulty of the sensor wire routing for the post-cat sensors. Is there any hopeful or more recent news here? I had read somewhere that you will disable the sensor if you cut and splice the wires. Is this true? My post-cat O2 sensors look so rusty! I believe they are original as I bought the car in 2001 and it's a 1997 model with 245,000 miles now. Just how important are those post-cat sensors?