Octavia 5E voltage supply fault, control unit 69

Hello everyone,

I have more or less successfully installed a Westfalia tow hitch on my Octavia RS, which was first registered in January 2015. As far as I can tell, everything is working correctly. I tested it with a bike rack, and the lighting functions as it should. Also, the trailer indicator appears on the radio, and the extra turn signal symbol is also displayed on the speedometer.

However, I have an error code 69 in the control unit that keeps recurring.

136203 - Power supply.
P2757 00 [009] - Interruption.
confirmed - checked since last deletion.

The wiring harness connected to the AHK control unit receives power from the fuse box via 4 connections to terminal 30, and from the BCM it receives 1 connection to terminal 15, 2 CAN BUS connections, and 1 brake light signal.

Unfortunately, I've reached the limit of my knowledge. Conversations with Skoda and Westfalia haven't helped me either. I received information suggesting that the error code indicates that the control unit is responding too late or not at all during the self-test.

I am very grateful for any help.

Best regards,
Fabian

Is the wiring harness in good condition and is it the correct one? I would initially focus my efforts in that direction.

Why I'm writing this:
My colleague installed a Rear-view Camera Kit (RFK) in his Golf 7. The original VW retrofit kit contained a faulty wiring harness.

I'm going to double-check everything again for safety. Theoretically, only one terminal (15 and 30) could be swapped. With any other combination, I would have more significant electrical problems. However, since all 4 of the terminal 30 cables are 2.5mm², and the terminal 15 cable is only 0.75mm², I'm almost certain that there's nothing mixed up.

The engine control unit (ECU) may be faulty, or there may be a wiring problem.

To narrow down the problem, I would intermittently disconnect each of the terminal 30 wires and observe which error occurs. The 4 cables are usually used as follows:
A) Trailer auxiliary power (Pin 9 of the trailer hitch).
B) Trailer socket plus (Pin 10 of the AHK).
C) Providing power to half of the lighting.
D) Powering the other half of the lighting.
I cannot say whether the STG itself is powered by C, D, or both.
To check the wiring, you could consider purchasing a temporary Erwin (siehe Fehlerdatenbank) access from VW for a day and downloading the SLP (Schematic Layout Program).

It's also possible that there's a missing ground connection or a lack of ignition power (and the control unit is being woken up via CAN, but it's missing its KL 15 connection).

I just removed all four fuses and read the FSP after each change. No new messages appeared. Here are three more photos for additional information. I've also included the measurement data blocks for the voltages.

What I found strange was that the main distribution box was always accessible, regardless of which fuse was blown. Is it not possible that it is powered by all 4 pins?

Access to Erwin (siehe Fehlerdatenbank) is available, and payment is not a problem. I'm not very familiar with the system...

In the screenshots provided, the charging line (pin 10 of the AHK. B) is clearly problematic. You should be seeing 11-14.5V, but you are only getting 0.8V. In my opinion, this input voltage will be insufficient for the control unit.

Was the fuse (specifically the one for charging) tripped in that circuit? Does each backup process involve lowering a different voltage?

Regards,
Guste.

I found the bug. It's almost embarrassing to write this, but here's what the problem was. You've now prompted me to carefully measure this entire cable line from beginning to end again.

"My car, from the factory, didn't have the wiring for the charging cable connected in the fuse box. When I installed the tow hitch, I connected the pin for that cable to terminal 22. Of course, there wasn't enough power available there. But for me, it made sense so that someone could eventually figure out what kind of device is connected there. To get power to that terminal now, I tapped into the wiring for the panoramic roof (which I don't have installed) using a jumper and a fuse holder. I, the idiot, miscounted and connected two terminal 30s in the fuse box instead of connecting one terminal 30 to the device. Nevertheless, thank you very much for your help." Sometimes, you still need a real push.

Thank you for the clarification.

Okay, so for posterity:
First, measure 4x terminal 30 and 1x terminal 15 at the control unit (STG) connector .