After returning from about a 150 mile trip, my wife's A6 was running a little rough while idling in Park. I want to share what the problem was so that people can put it on their list of possibilities when they get the same code.

The next day I plugged in my VAG tool and got these codes:

16840 - EVAP System
P0456 - 002 - Very Small Leak Detected - Intermittent
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 008 - - Intermittent
16686 - Cylinder 2
P0302 - 008 - Misfire Detected - Intermittent
17544 - Fuel Trim: Bank 1 (Add)
P1136 - 001 - System too Lean - MIL ON
16486 - Mass Air Flow Sensor (G70)
P0102 - 002 - Signal too Low - MIL ON
17546 - Fuel Trim: Bank 2 (Add)
P1138 - 001 - System too Lean
Readiness: 0000 0000

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Address 02: Auto Trans Labels: 01V-927-156.lbl
Part No: 4B0 927 156 FF
Component: AG5 01V 3.0l5V USA 1213
Coding: 0001002
Shop #: WSC 02334 785 00200
VCID: 70E41D2F60B8323

1 Fault Found:
18265 - Load Signal
P1857 - 003 - Error Message from ECU

-------------------------------------------------------------------------------
Address 03: ABS Brakes Labels: 4B0-614-517.lbl
Part No: 4B0 614 517 H
Component: ABS/ESP allrad 3428
Coding: 06497
Shop #: WSC 02334
VCID: 26483B77C2FC2C3

1 Fault Found:
18265 - Load Signal
P1857 - 35-00 - Error Message from ECU



The short story is that I had added oil when we stopped for gas and stuck the oil cap back in place but didn't tighten it. I thought I had smelled something later on but wasn't sure. Must've been oil that escaped past the cap that got onto the exhaust.

After checking the codes, I started the information hunt and read a recommendation in a post to look for vacuum leaks caused by things like splits at the ends of vacuum lines.

When I looked under the hood, I saw some slight oil stains and realized that the top of the oil cap was not in it's usual alignment.

I knew then that I had a vacuum leak with the cap loose and figured that was the source of the problem.

After tightening the cap, I cleared the codes and none of them have returned.

Obviously there are multiple reasons these codes can be thrown, but less obvious to me is that something as simple as a loose oil cap could cause so many seemingly unrelated codes at once.

Schaeffer

 

You never know how long some of the codes could've been stored for in the ECU, so not all of them necessarily were caused at the same time.

Nonetheless, a very good pointer.

 

Per: The 28F2/J1 service campaign on your 3.0?

Leaving the oil cap loose will cause an issue with the crank case ventilation system and strange idle condtions too, I have noticed on both of my cars when I remove the cap while running.

I agree with with GTA driver that some of those codes could be old non-hard latch. So did the CEL come on after the oil fill or before?

 

Good point. No CEL before I added the oil though.

 

The miss-fire codes might suggest coil pack failures on the horizon.

 

I replaced all six of the coil packs about two years ago and I would hope that they have a longer useful life than that, but based on the failure rate of the old defectives ones, maybe I'm just dreaming.

There are still no new codes and it's all good but I'm going to do a scan just to see if there's anything pending.

Schaeffer