MP4.2+6.0

Speculating about how two failed together won't fix them. I take codes at face value, and other than the ECU, I don't think the wires otherwise cross or end up at the same connectors. I would replace $80-90 O2s (Bosch OES via places like Amazon) long before I start painfully diagnosing complex ECU wiring harnesses or changing out big bucks ECU boxes. I've since notice a bit better mileage post 02 change (and a bad exhaust cam sensor that popped up in similar timeframe), so there were some follow on savings.

With your fuller scan, I see elsewhere you are over 180K miles (300K km+). Consider yourself lucky. I had two of my four pre cat ones fail, both on the same side, within a few thousand miles of each other, if I think back to when I first had codes an an occasional CEL. I'm at 103K miles now. I was in denial for a while about it couldn't be two, must be unrelated, maybe its a wiring thing, etc. More generally and from other posts I'm seeing, at least on the more complex four pre cat O2 sensor motors (w12's and S8s), they may only have a life of about 100K before issues start arising. Fortunately unlike all the (erroneous) internet lore about the W12, I found the pre-cat sensors fairly easy to pull, including the one farther back (B2S1 or B4S1) on a W12. My old 2000 A6 4.2 O2's were actually much harder to pull--basically had to literally get on top of the motor under the hood and quasi bling reach deep down with a wrench and try to loosen. It's the post cat ones (the S2's) that are the apparent W12 nightmares, but those typically have low failure rates.

aTOMic

Thanks MP. Re the ECU, the replacement procedure on these cars is absurd! They must be highly ranked among stolen car parts. They are plastic-welded in and it's a long, crazy procedure to remove the old one! I didn't even read the official install instructions! I agree O2 sensors need replacing but it blows my mind that they could fail simultaneously, unless they rely on each other somehow.

Re the topside O2 sensor removal on mine (4.2), Bentley says not only that one has to be on top of the engine, but the entire intake manifold had to come off! I sure hope there's a workaround, I think I'd remember that writeup if someone on AW did an O2 sensor that required that much dissection!

Also I don't understand why the engine freeze-frame data is not recorded. Seems like the most important module. The rear doors' data is there but not the engine?

Well, off to search for those answers.
-Tom

aTOMic

Took kids to school ~23 mile round trip this AM, no DTCs (except the engine mount) at all.

I wonder if bad gas could've, partially at least, caused this? I got gas in both cars at the same place within two days last week . My Passat has been idling noticeably rougher; in fact I've been meaning to scan it since last week. I put a bottle of Techron in each car, the A8 has been through that whole tank. Last night filled up the Audi (21 gal) and no codes since then. Granted it's a long shot, but wouldn't it be nice...

MP4.2+6.0

On removing the ECU, I can't be sure. On my W12 even w/ the twinned ones, it's pretty easy actually. Relative to my old 2000 A6 4.2 (which was pre code protection), it was harder, simply because it was wedged into that plenum area on the drivers side very jammed together with a big relay and electrical box. On my W12, they sit in plain sight once the plenum cover comes off, though one is partly tucked under the windshield edge. I have moved them aside before in order to get access underneath to remove the heater control valve block. Yes, Bentley makes you think its a million steps of security stuff but not my experience. The security stuff is there AFAIK know more to slow down tuners. After Audi ended up replacing a ton of turbos under warranties on 2.7T S4's and A6's both due to poor oil cooling and often after owners chipped them and then played dumb when the turbos blew (but at the same time they seemingly tried to keep a lid on the "reliability"/design problem) they redoubled their efforts to lock a bunch of this stuff down a lot better. Popping up in some ways in the current era for new Audis where the sense is they are going to lock out any access to modules except when tied to the German computer mother ship, like an ECU or TCU flash already is.

On pulling O2's I can't be sure, but its not intuitive to me why pulling intake manifold helps much. It does get various hoses and wiring out of the way along the way, but of course you could just undo that stuff separately. On my old A6 4.2, when I pulled a head the second time to deal with an oil burning issue, at 145K I realized when the O2 was just staring me in the face to change it for the $80ish dollars pre emptively, all the more since that side was running more oil out the exhaust. Took about 3 minutes literally. Pre 2004 though Audi only had the further downstream cats before the emissions rules tightened (why they only sold RS6's in USA as 2003's), so the D3 4.2 layout may be a fair amount different.

First step is just get in there with a flashlight and poke around to try to spot where they are and what is in the way. On W12's it comes down to just pulling the twin air boxes and then they are in plain sight. See first attached picture. Front one stares you in the face. The rear one is in the shadows until you look closely, and I have the wrench on it. The rears are a tight reach between the frame rail and the exhaust manifold. Could be about where the 4.2's now are physically since you only have one manifold per side instead of the twinned ones like the V10's and W12's. But the W12 is a wider motor than the 4.2, and they may have had them pointed more "inward" on the 4.2, which probably hurts rather than helps with my W12 technique. Even though I could not get my hand in there at all because of the very tight space to the frame rail--by a good 6" of reach--I could get to it with a 7/8" crow's foot wrench mounted on a 12" or so extension in 3/8's drive. 7/8" was better than metric to allow a little play for the slight off angle insertion of the extension (you can see it in the picture a bit), and the O2 is sharp hex cornered stainless so very resistant to burring. The second picture I snapped to show what seems at first inaccessible is actually simple with very standard tools. Once I cracked it loose the first tiny bit of a turn, I knew I had it. After a fraction of a further turn, I just used the wire and spun it counterclockwise to get out--I didn't care if the wire got messed up. The new one has anti seize on the threads so went in almost all the way with very little force on the wire once I got it started in the threads. A few fractional turns on the crows foot and it was tightened down fully. Debunks the whole all over the web/chatrooms/Bentley type W12 think of, "step 1, pull motor" said indiscriminately no matter which sensor. Post cat, perhaps yes, pre-cat definitely not and actually pretty darn easy. Now BTDT and field proven. It took literally as much or more time to deal with remounting that infernal intake rubber air hose clamp we are all saddled with than to get the rear 02 out and in once I focused on it.

Last, on your VAG COM experience, not sure why. FWIW, when I am on a focused issue like an engine or tranny error, I have recently taken to just snapping a picture of it on my iPhone. VAG COM and Bentley reside on an obsolete XP laptop for me, so easier to just skim thru my pics via screen shots than painfully resurrect old scans (if I saved them) and then move across computers via e mail or a USB stick. Helps too if I am in the middle of a VAG COM session on my already small 10" screen old Sony XP and want to confirm the prior codes were the same or different without starting to open more windows and such.

MP4.2+6.0

I was thinking of bad gas as a possible to explain commonality. But at 180K miles, they are probably on borrowed time anyway and you likely aren't truly optimized for both performance and economy.

aTOMic

Thanks again for the thorough replies. I cannot see anything from the top until I get the air intake and other hosework out of the way. I'm surprised they can't be reached from the bottom.

I email scans from MY old crappy XP machine too. I use the phone camera occasionally for remembering how something came apart, but good idea. Thanks for the tip on the 7/8" "wobble socket", might come in handy!

PaulW

To get at that one, I had to use a 7/8" open end wrench. A O2 wrench wouldn't go on it. The biggest PITA is the electrical connector, getting it disconnected and rotated as you unscrew the O2 sensor.

On a lift it would be easy. As I recall, I took the electrical connectors for all the O2 sensors off (one bolt) also.

Panelhead

Were you getting gas on Gallatin Rd?

aTOMic

YES! Next to a chick in a Maserati!

Whatcha driving?