How can I diagnose the root cause of a 54113 error?

Start by checking the status of your backend servers. Use tools like `ping`, `telnet`, or `curl` to verify network connectivity and that the backend is responding on the expected port (usually 80 or 443). Examine backend server logs for errors. Also, check Varnish's logs (`varnishlog`) for more detailed information about the failed backend connection attempts. Finally, verify DNS resolution is working correctly.