Fremdradio anstelle des Originalgeräts eingebaut

The vehicle's original stereo was replaced with one that was not designed for it.

On a modern vehicle the head unit is not an audio device connected to speakers — it is a module on the data bus, and it is expected to speak. It reports its state, acknowledges messages, relays steering wheel controls, requests chimes, adjusts volume to road speed, switches to the reversing camera, and joins in the negotiation about when the vehicle may go to sleep.

A replacement unit either does all of that through a proper vehicle-specific interface, or it does not do it at all. A generic adapter or a bare wiring harness leaves the vehicle sending messages nothing answers and receiving replies that make no sense — which is why the resulting codes name the network rather than the stereo.

Two consequences are worth stating because installers rarely mention them. The network may never sleep, so the battery goes flat over a few days and gets replaced twice before anyone looks at the radio. And the vehicle's own diagnostics become unreliable, producing faults in systems that have nothing to do with audio and absorbing a great deal of somebody's money.

On newer vehicles the same cause appears in a second form: an original unit taken from another car of the same model. These are frequently tied to the vehicle's identity and on many designs to the immobiliser, so they must be programmed to the vehicle rather than plugged in.

The repair is the interface module the installation should have included, or coding the unit properly — not chasing the symptoms it produces elsewhere.