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Verify physical network cable connections at the TCU (Thin Client Unit) or PCU (PC Unit) and check that link LEDs are illuminated on network ports.

Test network connectivity by pinging the remote PC IP address from the HMI commissioning menu (Startup > Network > Diagnostics) or using the service screens.

Check network switch, router, and VPN gateway for proper operation. Verify firewall settings allow required ports for Siemens remote diagnostics (typically TCP ports 102, 8080, and VNC ports).

Re-establish the remote diagnostics connection using the proper disconnect/reconnect procedure through the HMI Service menu or restart the remote service application on both ends.

Check the timeout settings in the remote diagnostics configuration (HMI > Startup > Remote Diagnostics > Settings) and increase if necessary to prevent premature disconnection.

Review the NCK alarm log and Windows event logs on the PCU/TCU for any related errors or crashes of the remote service application occurring at the time of disconnection.

Restart the remote diagnostics service on the SINUMERIK control: Navigate to HMI Service mode, stop and restart the VNC or Teleservice component.

Restarting services may temporarily interrupt other remote connections or background processes.

Verify that the remote PC has the correct and compatible version of SINUMERIK Operate or Teleservice software installed and update if necessary.

Verify that valid remote diagnostics licenses are installed and active on the SINUMERIK control by checking HMI > Startup > License > Overview.

Check the access protection settings (Access Level, User Management) to ensure the remote user has sufficient rights for remote diagnostics and that the password has not expired.

Review the system security logs for authentication failures or policy violations at the time of disconnection. Navigate to Diagnostics > Logs > Security.

Acknowledge the alarm and re-authenticate the remote connection using valid credentials with appropriate access rights for remote diagnostics operations.