mtfarmer4848

Okay internet, do your thing.

Quick story time to explain how I got where I am: months ago, my A6 threw P0056 and whatever the code for the B2S2 O2 sensor is. Basically, I assumed that the O2 sensor died, so I drove on it for a while until I could fix it.

On Monday, it ran like crap leaving work. It threw a P0101(?)(I forgot to write it down) and said I had a MAF sensor problem. Limped it home to work on it this weekend.

Today I grabbed a can of MAF cleaner and a Bosch 16978 O2 sensor to finally put this CEL to bed. Cleaned the MAF and changed the O2 sensor, and at first life was good, other than the P0056 code didn't go away. Then, after it warmed up a while, it threw a P0302 code and had a definite cylinder 2 misfire. Turned it off, cleared codes again thinking maybe it was confused, and started it again. Misfire didn't come back so I assumed I was on the right track.

So then I started down the heater circuit rabbit hole. I tested the bank 1 sensor with a test light on pins 3 and 4? (whatever the two pins are that coincide with the white wires going to the O2 heater) as it was working properly. Light flashed at the article I read said it should, and the same happened on the bank 2 socket. The bank 2 socket also had a good ground on pin 2, but I was having voltage issues to begin with. The new sensor was pinned at 1.13V, but that seems to have resolved itself as now its around 0.45V? It doesn't fluctuate as much as the bank 1 sensors do though. Maybe my new sensor is junk given the P0056 and inconsistent voltage readings? Let me know your thoughts.

Oh you thought that was it? I wish. So, ignoring the P0056 code, I decided to go for a test drive to see how things went. Made it a few hundred feet and the thing went nuts. Now I'm showing P0300, P0301, P0302, P0161, and P0130. The P0056 may have been a pending code again but it didn't store. I will note that I haven't disconnected and reconnected the battery after testing the sockets for the O2 sensor sockets on both sides (i did unplug and replug both sensors while it was running to test the strobing of the heater circuit). I gave up for the day but am going to pull cables tomorrow and see if the thing is just super confused. Do you believe that the MAF issue is actually resolved after the cleaning or is it somehow contributing? Am I missing something obvious? TIA for your help, whoever can crush these issues for me will be forever in my debt.

georgeb944

You can monitor your fuel trim and oxygen sensor readings in VCDS.

mtfarmer4848

Okay, new day, new test drive. Started by pulling battery cables off and letting it sit a while with the key on to make sure I discharged everything. Hooked the cables back up and scanned the codes again before starting. P0056 was a stored code but nothing else. I wiped it and went for a drive. I'm attaching some pictures to show the four codes it popped up with, as well as my O2 sensor voltage trends, and short and long term fuel trends. Note that I don't have access to VCDS as I don't have a laptop, but I'm using a Wi-Fi style OBD2 reader and DashCommand to read it. The trends I mentioned are on the right hand side of each category (I believe they read as peak, average, and low point).

mtfarmer4848

It may be worth adding that the O2 sensor I just replaced is Bank 2 Sensor 2, that was the Bosch 16978 part #.

georgeb944

Off the top of my head, I would say that Bank 2 Sensor 2 is bad, stuck at the bias voltage, not fluctuating as it should. It may be one of the recent spate of counterfeit parts.

If you bought it cheap off ebay, it quite likely is not Bosch.

Just because it's new doesn't mean that it is good.

mtfarmer4848

Thanks for your feedback. I just picked that sensor up at O'Reillys yesterday, but it had a lot of miscoloration on the exterior of it which makes me wonder if it was remanned? Ill try to disassemble it and get it swapped for another one. Does that part number seem right? They had a universal fit sensor but that sounded like a bad idea.

Would you assume that the B2S2 sensor reading incorrectly would lead to bad codes for the other bank? Feels like that shouldn't be the case but the B2S2 sensor is the only part I changed, other than cleaning the MAF.

georgeb944

Hard to say... These engines are fairly complicated and there may be interactions that I may never know.

Trust the Bentley manual and the Ross Tech wiki... best sources of information around for these 20 year old cars.

mtfarmer4848

Well, new day new problems. Replaced the O2 sensor with another brand new one at O'Reilleys and pulled another MAF from the junkyard. Still having similar codes to before, see pictures below. The B2S1 is doing really weird things, before the other sensors start really changing much, it's bouncing from 0.047 to 0.9V. New sensor is still pegged at 0.45V. Feels like a communication issue potentially? The short term fuel trims on all 4 sensors is swinging +/- 25% without much reason behind it. May be time to wave my white flag and have a shop dig into it.