Fendt 1.2.7F:

Inspect the engine wiring harness between the A051 Engine ECU and injector valve 4 for chafing, melting, or physical breaks.

Disconnect the plug at injector 4 and the A051 ECU. Check for pushed-out pins, corrosion, or moisture ingress.

Use a multimeter to verify continuity on the two wires running from the ECU to injector 4. Test both wires for a short to ground or a short to voltage.

Measure the internal resistance of injector valve 4's solenoid. If the resistance is out of specification (open or shorted internally), the injector itself is the cause of the code.