Fabia: No diagnosis possible when factory radio is connected!

I'm having a problem. I bought a Symphony radio for my Fabia (factory radio), but now I can't access the diagnostic menu when the radio is connected. However, if I temporarily disconnect the diagnostic pin (3), I can access it immediately. I don't have this problem with any other control units! What could be the issue? Vehicle: Skoda Fabia Classic, manufactured in 2002. The diagnosis was performed using VAG diagnostic tool. When the engine is running, everything works, and I can access the instrument cluster.

Radio shorts diagnostic line -> defect. It happens often.

Do you mean the radio is broken???

No, not necessarily; it might just be that the pin assignments are incorrect. So, let's check.

The pin configuration is correct. I just swapped the ignition and constant power connections because the radio kept forgetting the code.