Read TDI error codes yourself...

Hello everyone,
I recently installed a 1Z engine in my VW bus. Now I want to read the error codes, because for some inexplicable reason, there's no control signal reaching the fuel shut-off valve.
I've already identified all the cables from the tree (gray/white, +, -).
Now the question is, how can I read the data myself? A friend suggested that I need to build a "small" circuit and solder a connector to it to connect it to my PC. Does anyone know anything more specific about that?
Sure, I'd be happy to help!
Greetings from Freiburg.
Tobi.

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