CWTech

Regardless I have the car back at Audi to diagnose what I'm almost certain is their **** up.

I did replace the MAF sensor which of course didn't do anything, so now I have to wait for the dealer tech to sort it out. Fingers crossed.

MP4.2+6.0

IF the reading is correct, obviously its too much air or too little fuel. Reading says it is one side only (bank 1; U.S. passenger side), so that tends to dismiss things that would affect both sides of the motor. Its a guess, but besides continuing to look for vacuum leaks and even cracked manifolds and such, next places I would probably go if it were mine would be climbing all over the combi valve set up on the passenger head (cuts between EGR when hot and air pump when cold; seemingly problematic on my prior motors that had them when I had elusive codes), whether the reading itself is the issue (as in bad forward O2 sensor), or an issue with what may be only one injector.

Indirectly, a spark plug pull might give you a clue on an injector issue if it doesn't show directly with VCDS type scans and tests. I haven't had to diagnose injector issues since the old CIS/pre electronic Audi systems. Thus with old school plug visual stuff, one that is more toward white/tan than others toward grey/black. Or a whole side that is toward the lighter colors suggesting a systemic issue, which would more likely be the O2 or one side (head) vacuum leak. Old school stuff like that can be quasi subjective though and takes a fair number of miles of running to see something meaningfully different on the plugs, assuming it isn't burning oil that could mask it anyway. Hopefully either dealer and/or some further motor visual inspection or VCDS/dealer equipment type testing on fuel or O2 shows something.

As a final add/thought, one I have never run down myself might be the EVAP (vapor) system. That is switched I think to pull the vapor out of the holding canister(s) periodically. I see those switched valves with medium sized fuel oil type lines under the hood at times, but always just leave them be and dutifully reconnect if I have to move something and they are in the way. If something is wrong with the switching or the lines or equipment beyond the switch, seems like that could pull unintended air and lead to a lean code. And if at or beyond the switch it might be only occasional, which seems like it is part of what you are describing. Remembering my C5 4.2, seems like those components were over on the passenger side of the motor, at least physically. Never diagnosed it though, so this is just a pointer to think about that possibility.

CWTech

Update: the dealer had the car a few days and said "We think the MAF needs to be replaced." Since I'd done that literally 1 day before bringing it to them, I told them hell no. They then suggested I drive the car for a bit to see if the CEL comes back on.

Today it did. Now I'm getting a different code, not lean condition but 19058 - Liner Oxygen (Lamba) Sensor B1 S1 Pump Current Trim Circuit.

Does anyone know if a vacuum leak might trigger this? I'm getting to the end of my rope and I feel like driving this A8L right up their ***.

MP4.2+6.0

That looks like a straight O2 sensor code to me. But, please post anything else about the code, or better yet a screen shot. If it does not mention a communications error, open circuit, short to ground, etc., I would change sensor next. You can also Google it and see what else you find. B1 S1 is 4.2 (U.S.) passenger side forward sensor--same as what you have seen before as far as that motor side. If it mentions anything electrical, from personal experience you also have to consider the sensor plug on the car harness side.

CWTech

Sadly I have no internet on my VCDS computer at the moment, so excuse the image:

CWTech

Just to recap, I've already replaced both upstream O2 sensors, the MAF, and PCV valve.

MP4.2+6.0

I think you may have the broken connector wire issue with this exact code in hand (and hence why its important for even the trivia in it). Total needle in a haystack stuff. The code is a little different than mine, but close; may just be a different sensor location on mine than yours; I haven't tried to decipher it to that level. Key are the "004" and the words "circuit" and "open". I think that is saying electrical in nature, NOT fuel or air.

If you send me a AW message with your e mail, I can send you a whole message string on this with another member who has been running it down, apparently also successfully. Fix would be splicing in a new connector and related terminals+wires on harness side where the passenger side forward O2 sensor plug connects. Had this kind of code stuff tying to sensors on my W12 and eventually 100% nailed it when I changed the connector. Some of the wires in the connector (the ones that don't power the heating circuit) are really thin gauge and apparently can break internally or have issues at the terminal to conductor crimp.

See also this post from a quick search I did on when I've seen this before: https://www.audiworld.com/forums/a8-...p2254-2868359/

CWTech

Thanks a ton for the feedback. I took a look and there it does look like the wire might have been pinched by something. I'm going to take it back to the dealer to look into the matter since they did the installation. I'll post again when I hear more.

CWTech

Long time since update but here is what was found.

It seems that I had 1 of the two fuel pumps failing intermittently. The Audi tech was able to monitor fuel pressure and could watch the one pump drop in pressure which was corresponding to lean conditions.

Apparently the difference in pressures was causing aeration in the fuel line which is why I was seeing a lean condition. The lean condition was occurring in both banks, although random chance had it flash in Bank 1 the first few times. There was basically a 1/8th chance for any cylinder to get the air bubble, so a 50% chance for either bank.

I had my independent shop change both fuel pumps and filters, and I've not seen the CEL again. While hopeful, I'm still cautious to say this completely fixed. I'll give it a few more weeks and then declare the problem resolved. Fingers crossed.

richard-tx

I think you are good to go.
I saw that same failure mode on a 2000 A8.