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Verify physical network connection - check Ethernet cable, switch port LEDs, and X120 interface connection on the NCU.

Test network connectivity by pinging the remote diagnostics server IP address from the control's commissioning mode (ping command in command line).

Check firewall settings on both the control (if Windows-based) and customer network - ensure ports 22 (SSH), 443 (HTTPS), and Siemens-specific ports are open.

Coordinate with customer IT department before modifying firewall rules.

If using VPN connection, verify VPN tunnel is active and stable, check for timeout settings, and restart VPN service if necessary.

Check available disk space in commissioning mode: navigate to Storage management and verify at least 20% free space on system drive.

Delete obsolete NC programs, log files, and archives from /user/sinumerik/data directories to free up space if needed.

Back up critical NC programs before deletion.

Check archive logs and temporary folders (/tmp or /var/tmp) for large diagnostic files that may be blocking new transfers.

Run file system check using commissioning tools or boot into recovery mode to repair corrupted file system structures if disk errors are suspected.

File system repairs require machine downtime and should be scheduled appropriately.

Navigate to Settings > Remote Diagnostics and verify the remote server IP address or hostname is correctly entered and reachable.

Check authentication credentials (username/password or certificate) - re-enter if necessary and verify with Siemens support that credentials are current.

Review SSL/TLS certificate validity - expired or untrusted certificates will block secure file transfers. Update certificates via Siemens support portal if expired.

Disable and re-enable the remote diagnostics service, then restart the HMI to reinitialize the connection parameters and clear any cached errors.

Service restart will temporarily interrupt remote monitoring capabilities.