Cause : External Fault comes when the drive received a signal from an external terminal triggering a fault condition. This means an external input signal that is programmed to trip the drive and prevent operation. 

The F0085 fault code on a Siemens Micromaster 420 (MM420) indicates an external fault condition. This fault typically occurs when the drive detects an abnormal condition or fault signal from an external source or device connected to the drive.

Here are steps to diagnose and address this issue:

1. Identify External Source:

   • Determine which external device or source is connected to the drive and causing the fault. This could be another piece of equipment, a sensor, a safety interlock, or a control signal.

2. Inspect External Connections:

   • Check all connections between the external device and the drive. Ensure that cables are securely connected and that there are no loose or damaged wires.

3. Review External Device Operation:

   • Verify the operation of the external device or sensor. Check for any malfunctions, errors, or abnormal conditions that could be causing the fault signal.

4. Analyze Fault Signal:

   • Use the drive’s monitoring functions and diagnostic tools to analyze the nature of the fault signal from the external device. Check the drive's manual or parameter settings to understand how external faults are detected and reported.

5. Check Drive Parameter Settings:

   • Review the drive's parameter settings related to external fault handling. Ensure that these settings are configured correctly to react appropriately to external fault conditions.