BMW fault code D904 diagnosis and troubleshooting

Fault code D904 in BMW vehicles typically indicates issues with the vehicle's network gateway, voltage supply, or CAN bus communication systems. The primary focus should be on checking the voltage supply and related wiring.

1. Inspect the vehicle's voltage supply system, including the battery and alternator output.
2. Examine the CAN bus wiring for damage or corrosion, particularly at connection points.
3. Verify the network gateway module is functioning correctly.

These systems are critical for proper communication between electronic control units in modern BMW models like the 3 Series or 5 Series. A professional diagnostic scan tool is recommended for accurate troubleshooting.