Control Unit Not Responding: Troubleshooting Guide

Hello fellow mechanics...
I'm having the following problem:
When I plug the interface into the OBD socket of my Golf 4 (and have also tested it with other vehicles), turn on the ignition, and connect everything to my computer, only the red LED lights up.
A connection to a control unit is also not possible!
I don't see any issues at the interface, so I'm ruling out a broken cable for now!
Here are two screenshots from VCDS.
"No. 2: Connected to vehicle via VCDS, ignition on, but no connection established with the control unit."

Connecting to vehicle, ignition on, establishing connection with the control unit.

If I disconnect the interface from the OBD socket and reconnect it, does that reset everything?

Can someone help me?

Thank you, Klühse.

Hello,

Quote:

A connection to a control unit is also not possible! I don't see any bends at the interface, so I can probably rule out a broken cable!

Okay, what could the following message mean when pressing the "Test" button in VCDS?

Quote:

K1: Short circuit to ground

Why don't you check that if you're not even looking in the manual to see what the red LED means?



What other vehicles have you tested?

Possible cause with destructive effect:
/viewtopic.php?t=24020&highlight=radioproblem

Other vehicles: Volkswagen Golf 3 (year 1997), Polo 9N, VW T4... the same issue applies to all of them.
I have to say that I successfully used the same interface with a different computer on a Golf 4 years ago, and I didn't experience any problems then!
I haven't used the interface in at least two years, and today I wanted to start using it again...

Short circuit to ground? Where could it be coming from? Could it possibly be a broken cable?
I'm ruling out the radio because the Golf 4 is from 1999 and the T4 is from 1998!

Thank you, best regards, Klühse.