gilee_8

Hey guys, I have a 2000 Audi A6 4.2L.
Since I bought the car a few months back, I been having a check engine light. And I been seeing 4 faults from vag-com. Both front O2 sensor and fuel trim too lean. (17539,17546,17521,17544)

So I know both B1/S1 and B2/S1 O2 sensor is dead and also cause a long term fuel trim too lean, so I finally bought 2 new O2 sensor and replaced it over the weekend and everything was fine after I reset the error code. Until tonight....
My check engine light pops on and I pull the code again and I got 2 code. Both are fuel trim too lean but no O2 sensor error.

Control Module Part Number: 4D0 907 558 S
Component and/or Version: 4.2L V8/5V G 0007
2 Faults Found:
17546 - Fuel Trim: Bank 2 (Add): System too Lean
17544 - Fuel Trim: Bank 1 (Add): System too Lean

Can someone please tell me WHY I get those error code? I thought those 'system too lean' are caused by bad oxygen sensor but how come now the O2 sensor code is not shown (coz I put in new O2 sensor) But the too lean code still show up???

Thanks guys.

Grapeking

the age on your car, it is likely they need replacing.

4Driver4

How did you know the old O2's were "dead"?

audi01a6

u have a vacume leak,lean,ck lines on back of heads for 2nd air valves,

gilee_8

The reason I know the old O2 sensor is dead because I got 4 faults code from vag-com.
2 is regarding the Front O2 sensors are too high in resistance.

gilee_8

So you saying the lean is related to vacuum lines. do any of you have a diagram of a 4.2L V8 engine's vacuum line??

Also, Audi01a6, what do you mean by the 2nd air valves?, do you have a picture of it?
Thank you.

Grapeking

have said was "check at the back of each cylinder head, there is a plastic line running between each of the Combi valves". That line can become brittle and crack.

Do a search onAW and you'll find many posts about the vacuum lines and Combi valves.

Oh, while you are at it, change the "subject" line on each of your posts, please. It makes for a proper thread that is much easier to follow.