A7 4K: Control units unreachable & data bus errors

Hello everyone.

I drive an Audi A7 4K 50 TDI, manufactured in 2019. The car has undergone several coding modifications (unlocks) and is a lot of fun to drive.

A few weeks ago, I had a sound generator installed. The vehicle was functioning perfectly before, with no error messages. The sound generator was connected (soldered) to the CANBUS on the right side of the trunk, as specified by the manufacturer (see attachment), and it was working. Since the installation, several systems have been working properly, such as:
- Adaptive driving assistant.
- Intersection assistant
- Warning: Do not leave the designated path.
-Audi pre sense
Lane change warning including exit warning and rear cross-traffic assist.
- Rear parking assist.
- Sensor-controlled luggage compartment unlocking.

Not at all anymore. For the installation of the sound generator, the rear bumper was removed, a hole was drilled, and it was slightly welded in place (see photos). According to the executing company, with appropriate safety precautions in place.

Numerous control units are no longer accessible, and there are numerous data bus errors. As far as I can tell, all of these control units are located in this rear section of the vehicle.

The Audi authorized service center (where the vehicle was subsequently taken) disconnected the sound generator, restored the status of the CAN bus line, replaced several suspected faulty connectors in the rear bumper area (without success), and finally decided to replace the CAN gateway and restore the factory settings. The CAN gateway was not identified as a source of error.

I would like to ask for your advice on how to proceed with troubleshooting. To be honest, I'm completely lost. What Audi is proposing seems unproductive to me and more like a shot in the dark?

Here's the scan taken immediately after the generator was installed. Unfortunately, I don't have a file like before, and it appears without any error codes.

If you need any further information or details, please let me know. Thank you for your help!

Regards,
Alex.

Hello,

I would like to measure what is happening on the CAN bus lines of the unreachable control units.
I.e. Check for short circuits or open circuits up to the gateway control unit, and also examine the signals using an oscilloscope.

During welding, things can naturally go wrong (it's a "mystery box" of potential defects), or someone might have drilled into the wiring harness, or something might be incorrectly connected/the pins on a connector might be misaligned.

Best regards, Rainer.

Hi Rainer,

Thank you for your response. I'll see if I can measure things as you suggested.

I'm ruling out the possibility that someone drilled into the wiring harness or that something was incorrectly connected/that the pins on a connector were rearranged, but I will double-check it anyway.

What is the worst thing that can happen during welding?

Regards,
Alex.

crove wrote:

What is the worst thing that can happen during welding?

Control units are broken.

You can test the inaccessible control units with VCDS by directly connecting them to the CAN-H/CAN-L lines. This will tell you whether the respective control unit is still functioning.

I'm leaning more towards a fault in the CAN bus wiring, possibly due to damage to the wiring harness, loose connector pins, or incorrectly pinned connectors that are causing a short circuit.

Best regards, Rainer.

The error has been located; it's hard to believe, but it was two connectors that were swapped during the reassembly of the bumper.

I stumbled upon this while I was checking the wiring for the lane keeping assist system's control unit. I noticed that the wire colors didn't match the diagram.

It's unbelievable that the authorized workshop didn't check this; they must have also inspected and approved the bumper, right? But the main thing is to just recommend a new gateway to the customer for 1500 euros, without any real reason.

Thank you for the tips; ultimately, they helped resolve the problem.

Great, thank you for the feedback!