A4 B8: System Outage - Bus Disruption?

Hello everyone!

I'm having problems with my 2012 A4.

The tow hitch and Audi Active Sound system were added 8-10 years ago, but otherwise, the car is equipped with standard hardware.
The AHK control unit has been temporarily reinstalled for testing, but not all functions have been fully reactivated. The problem still persists.

The car is driving normally, but then, out of nowhere, all the error messages that the instrument cluster can display start appearing.
Glow plug indicator, ABS (Anti-lock Braking System), ESP (Electronic Stability Program), parking brake, transmission fault, airbag warning light, tire pressure monitoring system.

Interestingly, the Audi Active Sound system either turns off completely or intermittently produces sound, then stops again.
Sometimes the planer won't start when stationary, but a few minutes later it works normally.
Despite the error messages, it is performing at full capacity.

Does anyone have any ideas about this?


Attached are two scans.
In the first scan, errors are present (various systems are unreachable). I created the second scan a few minutes later, without making any changes to the car or moving it.




Best regards,
Michael.

Hi,

I would obtain the wiring diagram and examine the path of the drive-related CAN bus. It's possible that the wiring harness is damaged, which could either intermittently disrupt the CAN communication or prevent individual control units from receiving sufficient power.

My search would begin near the ESP (Electronic Stability Program) and the engine control unit.

The control unit for the trailer hitch is likely not at fault.

Good luck.
Rainer.

Thank you, Rainer.

The MSG was out yesterday, and it looks perfect.
Everything in that area is dry, including the water tank.

Last year, the car experienced water damage due to a leak in the panoramic sunroof.
Possibly, these are still lingering effects.

We will be checking the drive control CAN bus tonight.
I am reporting.

Here's what I've noticed when comparing the logs:
In the initial scan, many control units (ECUs) are not reachable (e.g., MSG; transmission). During this scan, some ECUs also report that the MSG is not reachable (e.g., brake 03), while further down in the initial scan, communication with the MSG seems to be established again. For example, the climate control unit 08 already reports "sporadic" MSG communication failures (meaning it was working again at that point).
The BnSTG 09 reports intermittent issues with the MSG (Message System) and airbag systems, and a permanent lack of communication with the trailer hitch (AHK) and fiber optic cable (LWL).

Airbag 15 is noteworthy because it doesn't list any subsystems (the six crash sensors) in the first log entry, but it does in the second. I can't explain how something like that could happen.

KI 17 and Comfort System 46 then report an airbag communication error (even though it was previously read), and MSG shows as intermittent. In short: The airbag control unit seems to have been offline for a longer period than the MSG.

The diagnostic interface itself reports the drive data bus as being permanently defective (not intermittently failing, but as defective).

With the STG 5C lane keeping assist system, the engine control unit unexpectedly becomes permanently unavailable.

How often does the error occur? Is it reproducible in the short term?
Besides a meaningful (but challenging) system check, I would disconnect the airbag control unit and test drive the vehicle (attention: safety risk). The missing subsystems there are striking to me.

Thank you as well for the suggestions.

It was not possible to reproduce it.
The error first occurred last year on the way to the hospital to give birth to my daughter (it would have been preferable if it happened on the way back ), then about a quarter of a year ago, and for the past four or five days, it has been happening sporadically, but actually on every trip.


However, we eventually found what we were looking for.
The (original Audi) retrofit wiring harness for the Active Sound System is connected to the CAN bus behind the glove compartment.
In this case, the two wires of the CAN bus were damaged.